Thank you all for confirming, this is the design of the firearm. It seems that the spring doesn't have enough force at the last 1/3 of an inch of travel to pull it all the way forward, only when I'm riding the slide home. In all other circumstances it goes all the way forward and locks. I have another firearm, a Walther CCP M2, and they used double spring to make it go all the way home, no matter at what point of travel I let go of the slide. In all fairness, however it is an inch shorter barrel and fixed one as well, plus tension is very close to my M&P, although it is a much smaller gun.