OK maybe this will help. My left flipper has been sticking every so often for some time now so I did some flipper testing with the pf propped up on the service rails. The problem on my game is the button itself. It can actually get stuck on the pressed in position. Maybe it has debris in it causing this. I thought I could feel a problem with the button while I was playing and this confirmed it.
Edit: Now that I think about it, you'd have both flippers stick if it was the right button. But maybe this helps someone else.