Almost all times in these posts, the operator figures out they somehow were placing their thumb on the slide lock, and in such a way as to depress the lock during recoil, so the slide would not lock back. If this is not the case, then you may have a defective mag follower, that is not raising high enough to hold the lock up. Check to see if all empty mags will hold the slide back when you manually rack the slide back. If so it is most likely option one. Not normally a spring issue, but could be a stuck/sticking follower. Clean the inside of the mag tubes and the followers, assemble with no oil. If during your next trip to the range, when you know your thumb is not causing the condition, and it happens again, start another thread. State in the thread that you know it wasn't caused by your thumb.
Last edited by robkarrob; 08-18-2013 at 08:27 PM.