Bottom case loose on MacBook (unibody)

I used the MacBook Bottom Case Replacement Program, as my 'old' bottom case became warped. So I replaced it with a new one Apple sent me. However, the new case which I fitted doesn't seem to be flush with the actual body of the MacBook. The rear right hand corner of the bottom case (under the bottom right of the display) is loose and there is quite a gap between the bottom case and the body of the MacBook. I've tried tightening all of the screws and replacing the screws countless times. Just wondering if this is a common problem regarding the replacement bottom cases, as this problem is very frustrating!