handlerer2 may have the solution.
If not, check the mount screws to see if one is protruding slightly into the bolt locking area, causing a "sometimes" situation. It's unlikely, but possible if the screw is not the correct one for that application.
People are like rifles. Some are tried and true, having great eyes, personality, and fun to be with. Others never seem to hit the mark with you. Still others go off half-cocked. Still, it's nice to know most of them.