Ok, reread the 1st post. Again! I wonder if inadvertently the shooter while griping the mag/magwell with left hand had pushed on the catch and caused it to move up (like it was catching the bolt) and bent it enough for it to have this issue? I would replace the catch, catch pin, spring and spring pin just to get rid of all involved hardware. Some oversized catches hit against the upper and doesn't stay in the down position. Try a new catch and do a temp install with the catch and catch pin only, so you can check for ease of movement with upper attached and bcg removed (so you can visually check inside also).