ORIGINAL: rc-sport
Or your not tightening the screw enough. I really crank down on the screw.
I have been known to strip out that plastic stopper more then once bearing down on that screw. I know, my bad!!! I have ended up using a 4-40 hex screw with a nut behind the plastic when I have strong armed that poor little critter. A bit of CA to help hold that nut while I'm getting it started.