How old is the M&P - could be the early production mag release issue. Easily solved. Call Wortners in Eastern Canada or MD Charlton in the West. Could be thumb on button, try switching the release to the other side (many of us leave the button on the opposite side, it's fast and clean to drop them with the trigger or middle finger). If as Tigrr suggests you don't hear an audible click seating an empty mag (with the slide locked back, even) check that the mag is seating, if it is seating but not staying put refer to the first line. Oh, and bookmark M&P.com - tons of useful stuff there.