I've got a relatively early full size M&P40 with 12 magazines bought at varying periods. I recently sent it in to have the slide stop replaced due to the right side tab having broken off at some point. Since I got it back from S&W, the slide won't lock back either after the last round or when I manually retract it with some of the magazines. I didn't notice this problem before I sent it in.
Of the 12 magazines, eight have a polished finish and four have a matte one. The matte ones work fine; four of the polished ones lock the slide back but the slide stop doesn't seat fully in the notch in the slide, and the other four just don't lock the slide back at all.
It looks to me like there could be three reasons for this issue - a) some of the mag springs are weaker due to age or design; b) the follower design was changed at some point; or c) the spring on the new slide stop is stiffer than the old one. I compared the slide stop tension on the 40 to the one on my M&P45 1.0 and it does seem stiffer, but the specs for each gun could well be different.
My guess at this point is that the problem is with the magazine spings. I think that the springs in the ones that don't work don't apply enough upward pressure on the follower to push the tab on the slide stop up to engage the slot in the slide. But since I don't know my ass from a hole in the ground about this stuff, I'd appreciate any input from knowledgeable folks.
Wolff has ten packs of their +10% M&P mag springs, if I need to go that route.