I think it could be from riding the slide lock when shooting. I got my mp22 and shot 300 rounds through and experienced it not locking back a hand full of times. The slide lock is very sensitive and if you even rest you thumb on it lightly it wont catch on the last shot. At the range I would focus on keeping my thumb away from the slide lock and it would lock back every time. When I was plinking around and doing rapid fire not really focusing on grip the slide would not catch. At first I thought it was mechanical or ammo problem.

I was looking at my grip and experimenting at home and having that big safety selector naturally points my strong hand thumb to the slide lock.

As soon as I can get more 22 to shoot I want to see if this is the case. I hope its just an operator error. On weak ammo I can usually tell if a shot was weaker than normal because it sounds and feels different.