I use a gravis gamepad in the game to do split screen multiplayer with my friend, since he is pretty new to PC gaming and he is much better playing games with a controller. (plus on a laptop, it's kind of hard to split screen with 2 people comfortablly on one keyboard)

How I got it to work was I went into the settings and set the player 2 controller to "joystick" and it detected it as a 4 button 2 axis joystick.
It was working PERFECTLY until the other day. For some strange reason, the right button on the d-pad won't make any of the characters move to the right unless I press it rapidly multiple times. It's like the game just suddenly thinks I am not pressing it, or that the button doesn't exist. It's very weird.

Now at first, I thought this was an issue with the controller itself, but I've tried it in MULTIPLE other retro games and It still works perfectly fine. I've tried Claw, Jazz 1 (thought it was a bit tricky to setup for at first), Cosmo's Cosmic Adventure, Wacky Wheels, etc and it works perfectly fine in all of those. So I know it's not the controller with the issue, it's the game. It's driving me nuts because we've been playing this together every weekend for the last few weeks and were so let down that it wasn't working properly. I've even tried uninstalling the game, removing all the files in the directory, and then freshly reinstalling the game; same issue.

Can someone please help me fix this? It's driving us nuts!
PS this is also only an issue on my laptop. I don't have this problem on my windows 98 when playing Jazz 2, but that doesn't have internet access so I can't get the custom levels, and ithe pc itself has its fair share of moments when the thing doesn't even feel like starting up. So it's not reliable and it's not portable.

I haven't got the faintest clue, though it still sounds to me like the controller is to blame.
It's possible the button contacts got dirty, corroded or some other mechnical problem that causes problems with the game to register button presses.
Again, I'm not sure, but it's very well in the realm of possibilities that Jazz relies on more tight timing for input to register than the other games you tried.
Do you perhaps have a different controller to test it with? Or you can bind the action in the game to a different button to see if it stops.

So it wasn't clear, but the issue is with Jazz 2?

It's only with the game though. I've tried it on my win98 and it works perfectly fine on there (though that pc is a mixed bag and only starts up half the time)

I've installed Jazz 2 on several win 10 machines and the controller ends up acting weird after about a few hours of playing. It's all on win10 machines and only jazz2. It's the weirdest thing. Yes. Jazz 2.
It works perfectly fine in Jazz 1 across all PCs I've installed it on.

What controller is it?

Gravis gamepad.
I've tried countless other games (Hocus pocus, wacky wheels, snes emulator, blake stone, wolf3d, commander keen, etc), and none of them are having this same issue as jj2.

Ah, I think I have an explanation.

Many programs have learned to treat joysticks as simple devices. Rather than seeing "Gravis Gamepad", they see "Four buttons, two axis." (You can see this when many DOSbox games boot in the status window.)

But Jazz 2 was actually designed with the Gravis in mind, and is a windows program. Now, I presume you've got a driver or patch installed to make this possible.

That's the part where it gets a bit complicated.

It might be worth checking the calibration of your Gamepad on your Windows 10 PCs. Are you using the gamepad with an game port adaptor? I presume you are. If you're using Windows 10, go to the search box and type in USB game controller, test the buttons and the control pad in the game controller setup utility. Maybe even try recalibrating the controller in Windows 10. Follow the onscreen instructions and see what happens. What kind of adaptor are you using the plug the controller into your computer?