Chopard Caseback Screws

[calmadarch]

I opened up my watch to put some grease on the o-ring and realize that some of the case back screws don't screw-in well. feels like the screws won't catch as if the threads are stripped. however, if i switch the screws around and re-screw, then it happens again but with different screws at different positions. What am i doing wrong? do i need better screws? or do i need to screw in the screws in some sort of pattern?

If the screws are stripped, where can I get screws like these?

This is the watch I'm talking about:

http://forum.replica-watch.info/vb/showthread.php/chopard-mille-miglia-gtxxl-chrono-27291

thanks,