You sure someone isn't messing with you ??  Loosening your screws ??
Out of....a LOT... of box calls I've owned, I have had a grand total of ONE with a loose screw.
Nearly EVERY production box I get, the very FIRST thing I do is check screw adjustmemt.  The one loose call I do have, I simply shimmed the hole with a couple toohpick pieces, like Paboxcall suggested.  Works well.