I wouldn't presume to know enough to say what is really at issue for your ejections other than I have constantly read around here and other forums that your gun often picks what ammo it likes.

Sounds like your M&P likes Italian. If you aren't aware, Perfecta 9mm ammo at Walmart is made in Italy. Many are convinced it is also made by Fiocchi and distributed by Tulammo. At $9.78 for a box of 50 that is all I tend to buy nowadays. A lot of others do too since the spot in the ammo case is often bare but I tend to take a look when I am in there for something else.

Hope this is helpful a little at least.