Maybe a derp question, but here goes;
Was helping a friend out with some gun stuff;
They have an M&P 1.0 Fullsize in .40S&W.

I noticed an odd issue where, when pulling the slide to the rear manually, it does not always lock back reliably with a magazine inserted. I would pull the slide back expecting it to lock, and it would go forward as if there was no mag inserted. However, with certain magazines inserted, it would lock back as intended.

Of course I immediately suspected a mag issue, or maybe even a worn slide release/worn release spring.

However, when firing, it locks back normally on an empty mag as it should, and IIRC does not seem to care which mag is in the gun. And this is even with weak Tula ammo.

Now, I KNOW it isn't one of us accidentally pressing the slide release, since we both had the same issues with the same mags when attempting to lock it back on an empty mag manually, and it still locked back fine when firing.

So I'm kind of stumped. I still want to say it's a mag issue, but... what do you all think?