I was going to suggest a sprung washer before I was beaten to it! Failing a sprung washer, I'd suggest that maybe the screws might be a little over-tightened and could well be bending the plastic out of shape.
I have had this problem when fitting my drive into my power tower, but luckily you can unstick the drive's faceplate and move that instead, so I know just how irritating it can be.
