How to tighten the bezel to the back case if it becomes loose !?

hello everyone, happy if repairs iphones answer me question. cracked case near power button on 3gs, day bezel little bit loose (about 1 mm) , isn't tighten case before. replaced case today , still same. bezel doesn't want "stick" case. careful when replaced case , screwing tight, 1 can't suggest me something?
sure there isn't dirt or other stuff between bezel , case , sure tightened screws...
here picture:
http://imageshack.us/photo/my-images/217/91269501.jpg/
thank you!.

 

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