If you've run more than a few Dungeons and Dragons sessions, you've probably run a bad one every once in a while. Sometimes though, it can seem that no matter what you do and how much you prep, that your players just aren't enjoying your campaign. Have you ever stopped to think that maybe it's not you, it's them?
Problem players come in all shapes and sizes. Maybe they're just not engaging in your story, and are spending most of your session on their phone checking the football scores. Maybe they're being a classic rules lawyer and picking you up on every single judgement you're making at the table. Or maybe there's tension between your players themselves, and the issues they're having are spilling over and effecting the campaign for everyone else.
Bad D&D sucks, and as DM it's basically your responsibility to address these issues as soon as you can. Here are a few ways you can try to either reduce the chances of problem players at your table, or to rectify these issues if they do occur.
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It's all communication
Communication makes most things in life better. D&D is no different. It takes a lot of trust to fully engage and commit in a D&D campaign; roleplaying characters and thinking about how they would respond to emotionally charged situations etc. If you're willing to trust people to this extent, by allowing them an insight into your thoughts and imagination, then you should equally be able to have a conversation with them about why some sessions might not be going great. Not only can open and honest conversation resolve a lot of issues with problem players, a lot of the time it can actually head them off before they ever come up.
Session 0 is super important
A Session 0 is a brilliant way to head off any rules lawyers before they become too much of a thorn in your side. During this session you get a chance to talk about your particular DMing style, and how militantly you stick to the rules as written, or if you like to follow the rule of cool. You can also introduce them to any particular homebrew rules that you like to include. That way they are forewarned before you even play your first session, and you can always remind them later down the line that this is your preferred playstyle, and it's up to them whether they want to accept it, or if they'd be happier playing at a different table.
As campaigns can often be in a state of flux, with players leaving and joining, it may not be possible to have a Session 0 with every player before they start playing in your campaign. That said, it's still a good idea to drop any new players an email with some of the most important things you covered during the Session 0 (even if that was years ago!). That can include your DM style and approach to rules and homebrew etc - again, giving you a chance to nullify any rules lawyers before they start. It's also worth checking in with them to ask if this sounds like the kind of campaign they'd like to play in. You don't want to waste either their time or yours by having a player at your table who doesn't really want to be there.
Communication doesn't stop at Session 1
Just because you mentioned certain things during a Session 0, doesn't mean that your players will necessarily pick up on them. You also can't foresee every potential issue ahead of time - and things will come up that you might need to address with your players outside of the game. Whether that's a technical issue regarding rules and gameplay, interpersonal issues with or between players, or some/ all or your party not seeming very invested in the game you're playing. Talking about these things is good!
Sure, awkward conversations can be... well... awkward. But like so much in life, once you take those first tentative steps, it becomes a lot easier. And doing something is usually the only way to actually resolve these issues anyway - rather than just hoping that your players are gonna start enjoying your sessions more, or that that one rules lawyer is suddenly gonna get a new job half way across the country and has to leave the game.
Even when my players seem to be having a blast, I make a point of asking for feedback on how they're finding the campaign every couple of months or so. This means firstly that I can ensure my players are enjoying what I'm creating for them and that they have a space to ask for any changes that they may want/ need. But also, it creates a safe space and gives my players the knowledge that I really do want to hear their thoughts. Going forward, this means that if something does come up down the line, I've already set a precedent that I'm open to their thoughts, concerns and suggestions - which hopefully means they're happier to come to me with small issues, rather than let them grow into bigger ones which might completely derail a campaign or destroy the goodwill among the party further down the line.
When just one player is the problem
A lot of the time, it's just a single player who is the issue, and their actions are causing problems not just for you as the DM, but for the rest of your players as well. Maybe the player is hogging the limelight and making every session about their character, maybe they're calling you up on every ruling you're making so each session just becomes a tedious rules argument, maybe they're being rude, disrespectful, or aggressive towards other players. Sometimes people just kinda suck, you know?
At this point, it's a bit of a judgement call, as you are the one who is gonna have a better idea of the specifics of the situation and the different interpersonal relationships at play. It might be best to talk to your other players first to see what their opinions of the player/ the situation are. It might be better to try to talk to the player in private first, before bringing anything to the group's attention. You've gotta make the call here.
In general though, if it's a relatively minor issue and isn't directly aimed at the other players (say, the problem player is just hogging the limelight) then I'd be more inclined to have a conversation with them privately first. They may not even be aware of the behaviour, and want the opportunity to remedy it. If it is more serious though and directly effecting your other players (such as the problem player being aggressive or picking on other party members) then I'd be inclined to talk to the party first, to get their opinions on the issue and see if we can find a way forward to present to the problem player.
Don't be afraid to kick players (from your table. Not literally)
If compromise can't be reached though, and the problem player continues to be (for want of a better word) a dick - then don't be afraid to kick them from your table. You're playing D&D for fun, and everybody's enjoyment matters here - so if someone is ruining it for everyone else, you're under no obligations to keep playing with them. Depending on the nature of the offence, you can either consider warning them and giving them a chance to change their behaviour - but for serious stuff like abusive, threatening or discriminatory behaviour, you are totally justified in adopting a one strike and you're out approach.
When It's not them, it's you
What about the occasions when the reason you and your players aren't enjoying your game isn't one of them though, but you? Well, then it's time for a bit of self-reflection. Again, talk to your players and see what their issues are, and then reflect to see whether their comments are valid, and whether it's something that you'd like to change or not.
Maybe they're annoyed that your judgements on rulings change completely week on week and you seem to have a very weak grasp on the rules. Well, in that case maybe it's a good idea to just get a copy of the player's handbook (*) or the Dungeon Master's Guide (*) and brush up on the specific rules that you're struggling with. I'm all for flexibility with interpreting rules to make sessions fun and rewarding - but if you're struggling with the real basics such as knowing how combat mechanics work, then you can see why your players might find that annoying. In that instance, a little bit of learning on your part might smooth things over considerably.
If it's something bigger or more fundamental though (maybe your players want to play a silly campaign while you want to do a serious one, or they want to play an on-sea heist adventure while you want to do a gritty campaign based in a post-apocalyptic city) then maybe you want to think about whether or not you're happy to accommodate them. You don't have to just bend to what they want if it's not fun for you too.
It's totally cool to just realise that you want to play different games, and they'd be happier with a different DM, and you'd be happier with a different group of players. Tell them this. If you trust each other enough to play together, you should trust each other enough to have this conversation. And what if they're a group of close friends that you've known for years and still want to see every week? Then just suggest doing boardgames or going to the pub instead of playing D&D. It'll be less work for you, and you'll get to enjoy each other's company, without the stress of playing in a campaign you're not all enjoying.
Final thoughts
Without wanting to sound like a broken record, communication and introspection are basically the most important things here. If it's a them problem then communication is the best way to resolve it. And if it's a you problem, then being honest with yourself about your strengths/ weaknesses and the type of game that you want to play are crucial in figuring out if there's a compromise to be made which will keep everyone happy.
And if there isn't, then you don't need anyone's permission to kick a player from your table if they're ruining it for everyone, or to stop the campaign entirely if it's not fun for everyone (including you). Nobody is entitled to the time you're spending prepping and running the game - so if it's not fun for everyone, just stop playing. Hopefully by being honest and talking to people before small issues can grow into big ones, however, this shouldn't be necessary.