Have not had a single malfunction on my M&P 15 Sport or OR. They have ran flawless. As a matter of fact, none of the military issue ones I used have ever had a malfunction.
Okay, I take that back, during exercises, the blanks they issue causes some problem.
Check your mags (check inside for the springs too) and you might find the culprit. If the mags are okay, check your ammo.