I had this problem. It turned out that the plastic of the flipper was rotating relative to the metal axle of the flipper. I assumed that I wasn't getting the axle tightened enough in the linkage, so I kept tightening until it snapped.
So, check to make sure the problem isn't internal to the flipper bat. (It was on a brand new flipper too, one more reason why I didn't think to check this).