Try this...make certain the chamber and all magazines are empty, then, with the slide forward, place an empty mag in the gun, now pull back ?smartly" on the slide and see it the slide catch engages as it should, thus holding the slide open. Try the same thing with all of your mags as well. (Including putting the +1 extension back on the mag if you want to try it)

The empty mag should cause the slide to lock back each time. If it does, and I suspect t will, the problem is more likely to be your grip. Make sure you "lock" your wrist and that your right thumb is not putting any pressure against the slide catch. (BTW...does this pistol have an "extended" slide catch by any chance?) If the slide catch is not engaging every time when using empty mags, there are a couple of other possibilities, but try the above first and report back on your results.

PS: What ammo are you using?