Make sure your cabinet switch isn't staying closed after you let go of the button. If not, check return spring, and remove clean and replace coil sleeve, flipper bushing, and coil stop, ah hell, just clean and rebuild that whole assembly.
Had a magnetized coil stop not to long ago.

Don't lubricate, they're designed to run dry.

Other thing is make sure your flipper plunger is lined up with (parallel to) the coil. If you angle it up to much (tightening it onto the flipper shaft too high) or to low, then you can get some binding especially after playing a while.

However, if you haven't rebuilt the flippers yet, go ahead and do that and make sure the plunger is lined up with the coil when you put it all back together.

I have the same issue with my upper right flipper on my D&D machine, I rebuilt the flippers and the strength is back, but it does on stick up sometimes. I found that the plunger doesn't always come all the way out of the coil. When i built the flippers i didn't change the coil and that is where this problem is in my case. If you can replace your coil and sleeve and i bet the issue will go away. I use machine lubricant on it for now and it works but eventually it dries up and need to add more (yes i know you shouldn't, but the coil is already in need of replacement), but will be replacing the coil at some point.

Well let's hope that is the fix, mine would work fine for days and then start sticking on the up stroke again. And the coil sleeve wouldn't come out of the coil so that's how I know I need to replace my coil.

Sounds like a pounded plunger

Are both springs the same strength?

snaroff

Heres a simple check, swap the coil stops with the other working flipper. If the problem moves with the coil stop you need to replace it. I would order a rebuild kit no matter what.