That means the drum key operated screws are either stripped, or (more likely) the holes they screw into are stripped.
It's an easy fix. I assume there is some space between the butt plate (where the straps go) and the shell. Just find some stop nuts at your local hardware store with the same threading as your drum key operated screws and you're in business. You'll probably need a second person to help you, because you'll need to use both hands to tighten the screws (one to turn the screw with the drum key and the other to hold the stop nut at the back of the butt plate with needle nose pliers).
There is a possibility that the drum key operated screws won't be long enough to pass all the way through the butt plate to grab the stop nuts behind it. If this is the case, simply replace your drum key operated screws with longer hex head or screwdriver operated screws of the same thread size.
I've done this before and it works fine; you just need a third hand to pull the strap tight.