I had a failure to feed problem a few days ago with a new M&P9. The FTF was always when there were 6 rounds left in the magazine. I was using the two magazines that came with the pistol and it happened consistently with one, periodically on the other. It looked like the follower was getting hung up on something in the magazine.

Found my answer here: New m&p 9 full ftf with magazine release for lefty