I also have a 1911 and an M&P45. I had some handloads last summer that exhibited the same problem that yours exhibited. The M&P wouldn't go fully into battery. I re-seated the bullets by a thousnadth or so and everything fed fine. I think that my handloads were just a tiny bit too long for the M&P. It may have been a throating issue and the difference between my M&P barel and my 1911 barrel was such that one fed fine and the other did not.
By the way, I was using my 230 grain home cast LRN bullets, lubed with Lee LLA and loaded in a mixture of range brass. My ammo has always fed and fired fine in the family 1911's, but the M&P suffered a hiccup.