Quoted from cscmtp:
Has anyone else ever had your flipper buttons just fly off the machine during a game? This has now happened 3 or 4 times (always the same side) and I have no idea why. And do the stand up targets require multiple shots before falling down or should they always drop at the first contact?
If the nut isn't tight enough this will happen, it could also be bent/mishapen causing it to come loose. You could try a little removable locktite (blue) or a small wood screw with a washer to keep it from turning.
The drop targets should fall with one hit, but there is a common issue where direct hits don't cause them to fall. This is fixed either by replacing the tension spring with a stronger one, or tightening the existing one by moving it down a few coils until it falls as desired.