The slide locking back with rounds remaining in the magazine is typically caused by the shooter's thumb touching the lever's engagement pad under recoil, lifting the lever so it catches the slide's stop notch. repositioning the thumb(s) can usually resolve this issue. (It's also not uncommon for folks shooting Glocks & Sig's, which also have the levers located back under the ball of the shooter's thumb.)
If you changed your grip (thumb position) and the "problem" disappeared ... that's a clue.
Another potential cause can be the use of ammunition with a really wide nose profile, which can catch the lever's inside tab under recoil, bumping it upward.
Another possible issue can be the healthy recoil forces developed by the .40 S&W cartridge, and the lever's wire spring not having enough tension to hold the lever down out of the slide's stop notch under recoil.
There's some normal amount of movement of the lever under recoil (something the factory calls 'parts flutter'), but the spring ought to keep the lever from lifting high enough to lock the slide back.
Last time I checked, the standard M&P 40/40c slide stop lever spring was painted light blue. (There are also unpainted and red painted wire springs that have been used since the gun's introduction, and the caliber can determine which spring is used.)
I was told that in some 40c's (and 357c's) the light blue wire spring might not have enough tension to keep the lever down during recoil. I'd imagine the use of stouter, lighter bullet weight loads in any particular 40c might also have an influence, since they'd often result in more recoil forces..
My 40c had this condition, and I was told I needed to have a heavier lever spring (meaning a new lever assembly, since the wire spring is crimped onto the lever). The heavier spring for the 40c is painted yellow. I was told at that time that they didn't consider the heavier spring necessary, or appropriate, for all 40c's (it might be too heavy in some particular 40c's, causing the opposite problem of not letting the follower lock the slide back on an empty mag). Just an occasional 40c.
Since I'm an armorer they sent me the new assembly with the heavier wire spring and I replaced it myself, but it's not a kitchen-table project. Best to let them check the gun and determine if it's needed, and then do it for you under warranty.
If you're sure your thumb isn't unintentionally bumping the slide stop lever, and the ammunition nose (ogive) isn't too wide, I'd call and ask for a shipping label so the company can check your gun. Be specific in listing the symptoms, as well as the ammunition you're using.
BTW, the current follower has a '40' molded in the top. It lifts the rounds better in the mag body, and also helps keep the bottom round more centered (away from the left side of the mag).
FWIW, I'd make sure it wasn't your thumbs unintentionally rubbing/pressing against the lever before changing parts, though. Might not be the gun that requires "adjusting", right?
Just my thoughts.
