The only thing that could cause the slide to lock back, is for something to move the lever upward. Normally, the only 2 things that would cause the lever to move would be either the Mag Follower, or a finger.
With Compacts and Subcompacts, It's very easy for one's thumb to inadvertently put upward pressure on the lever during recoil. It's an easy cause to overlook.
Lately, after 3yrs of shooting my Shield, I've had just the opposite problem with my thumb, that popped up out of nowhere... I'm periodically putting downward pressure on the lever, causing mine NOT to lock back when empty.
Go figure!