If your student has a USPc with a std/sm release and dropping the mag inadvertently, I’d say that’s unusual (bot not impossible).
Also why my P2000/USP9c have the std/sm mag release.
I experienced this issue nearly every time while shooting a family members VP9 Southpaw. It was not a problem shooting right handed.
I actually would be very interested in a VP9L if not for the unavailability of a shorter right side lever for this pistol. (I don’t modify my guns apart from OEM items)
I have actually installed right side short levers on all my P30 guns because of the issue shooting left handed, caused by my grip.
I can confirm my thumbs forward grip has caused the VP9 slide to not lock back on the empty magazine.
None of my magazines are out of spec or show any other unusual descrepencies that have been mentioned in other posts.
What I figured was happening is the meaty part of my hand, between the thumb and the wrist, was applying enough pressure to prevent the slide lock lever from popping up and engaging the slide.
I haven’t carried a SIG in a decade, carry Glock at work, and the VP9 family was new to me. It came to light as another instructor was watching me and when I was going through long strings of fire and reloading drills, did it occur. Not always, but a few times out of maybe three dozen.
This is causing me to change how I grip the VP9, which means I’m not carrying the VP9 any more. I plan to start more training with the VP9 and hopefully it’ll work out.
My other thought would be either a factory part that changes where the lever sits, or how it’s shaped. If it extended out more, maybe I wouldn’t smother it?
Flag Day Joe
The Cold War didn't end, communism still lives and has come to America.
My first VP9 was my first semi auto pistol. After a cumulative 35K rounds through all on them, I never once have had this issue. It's not the gun is all I'm saying.
VP9 - times 4
HK45 V1 German Made
HK45 V7 US built
HK45 V1 US built
HK45C V7 OD
HK 45C V1 German Made
When I was new to my P30 SK I experienced the same failure to lock back. Fixed my grip, no longer an issue. I also experienced dropping mags with the P30L. For some reason I had a bit of difficulty adjusting to the full size grip. Again, it just takes practice. Gotta put in the work. I like the USP mag releases because they’re a little smaller. Just keep working on it.