Out of the box my M&P Shield 9mm EZ would have a failure to feed about a third of the time.
The cartridge would get stuck about half way out of the magazine.
I polished the feed ramp using a rag wrapped around a pencil.
I also used an emory board to sand the feed lips, since it seemed they were rough and stopping the cartridge from leaving the magazine. I then polished the feed lips using a rag also.
BTW, the polishing was done using Brasso. Got to love that stuff.