My newly-acquired magazines have never locked back the slide after the last round...not sure why. A spring in there? Ideas?
Ideas? Yeah, same as I commented in your other thread about it.....9/10, your thumb is riding the slide release.
I finally got to run some mags through my 43X yesterday.
First, I tried my 43 with new Strike Industries mag extensions. Slide didn't lock back, so I immediately blamed the extensions' use of the stock mag spring, and figured I would need stronger springs to get the extended mags to work properly. I never had that problem with the stock mag bases in place.
I then switched to the 43X with stock mag, and the slide didn't lock back on that, either. Being a brand-new, stock Glock, I figured the problem was me instead of the gun or mag; so I shot the next few mags with a "flying thumb", and had 100% proper function. The buddy I was shooting with ran a few mags through it, too, and had no issues. The function with the stock mags and the Shield Arms mags was the same. I didn't think to pull the 43 out of the bag to try the same techniques, but I suspect I was riding the release on that one, too.
The only other pistol with which I've had a thumb/lockback issue is my Gen4 19FS. I was experimenting with the backstraps, and with the largest backstrap installed, my thumb would put upward pressure on the bottom of the extended mag release, causing the slide to occasionally lock back when it shouldn't. This only happened with the large backstrap installed.
Millie, try shooting a few mags through your 48 with your right thumb stuck out like you're hitchhiking. Based on my experience, I'm agreeing with
@Derek8404 .