Could be lack of magazine spring pressure, but could also be caused by the front of the magazine follower canting downward.  Maybe try some non-tilt magazine followers.  Other possible causes might be dirty or dry rifle, so for process of elimination try cleaning the bolt carrier & chamber thoroughly, then lubricate the 7 bolt lugs and rails of the bolt carrier so they are nice and wet with a quality lubricant like Slip200o "EWL" Extreme Weapons Lube".  Any idea of how many rounds you have through the rifle, as if you have quite a few then you might consider changing out the action spring (buffer spring), as they become weaker over time.        

CY6
Greg Sullivan "Sully"
SLR15 Rifles
TheDefensiveEdge.com
(763) 712-0123