[WWWJD]

On my last few outings, my M&P has failed to lock open on the last round fired. Last night I tore everything down (way beyond field strip; down to the last pin) and cleaned to make sure the slide release lever moved freely, no gunk anywhere (or over-lubed) in the trigger group. Tore down the mags and cleaned them out for the same reasons.

Still, there's a 50/50 shot of the slide locking back on an empty mag when you rack the slide. When I remove the slide and insert a mag, the slide release pops up fully as it should. However, when I push the magazine follower down into the mag and then release it, the slide release only comes up partially most of the time.

Are the mag springs getting weak? This pistol stays loaded, and I'd say that both of the mags that I have, have gotten the same amount of time being loaded for long durations. I can't think of another reason for this to crop up after about three years of service.