Physics colliders are shifting?

I'm putting together a pinball project and I have a nagging problem.

When I first run the game, the ball bounces off and is stopped by the colliders on the flippers just fine. Over time however, the ball collides closer to the flipper (geo) and eventually slips inside and through them. This seems to be a progressive effect.

I have tried using box and mesh colliders on the flippers. It almost appears as if the action of using the flippers or being hit by the ball is causing the colliders to shift.

I have animate physics on the flippers.

Has anyone else experienced something similar? Any suggestions? I'm wondering if I may need to set up a particular hierarchy of objects/colliders to get this to work.

 

I was hoping one of the physics gurus that frequent this forum would be able to help. But this post seems to be drifting unanswered down the ranks faster than an iPhone app with an expired release date.

Any ideas at all?

thx!