It sounds as if the threads are stripped. The rear trigger guard only screws into plastic. My suggestion would be to take some epoxy and fill the screwhole, and then drill out a pilot hole and screw in the rear screw. You don't need to torque down the rear trigger guard screw more than necessary to hold in the trigger guard and to not fall out. It should not be torqued to 30 inch pounds, etc... Hand tight will suffice.