Yeah, mounts can do that, if the mounting screws are continually loosened by vibration or shaken loose by rattly semi-auto actions. I've had a couple of Minis that required the mounting screws to be checked regularly and frequently for tightness. This was the case even when using the Ruger rings.
If the screws are not loose, why do you feel that it is the mount rather than the scope that's failing to hold zero? Is the scope a Walmart special?