Good to hear that :-)
First thing to check is if slide locks when you:
1) empty magazine and make sure chamber is empty
2a) quickly rack slide back and release
2b) slowly rack slide back and release
If 2a) does not work but 2b) does, maybe magazine spring is too weak to push follower up and lock slide. This can happen when slide moves quickly (as when firing), compared with slow operation by hand. If slide locks in both cases, try to make it faster and faster to see if there is a change.
Next possible reason is that blowback is too weak. It may allow ejection, but not go far enough to lock the slide. It hapens on my models and it's random. Except for cleaning and care I don't think there is a good answer for worse caps though.