It sounds like it's short stroking. The question becomes: why?
First, let's make sure we've eliminated mags. This does happen with all mags? Surplus, new, metal, polymer?
What is the ejection pattern like? Are fired cases dribbling out or are they flying? If they're dribbling, the BCG is going slower than it should. Could be under gassed, dirty chamber, bad rails, a number of things. If it's throwing empties, but still short stroking, something is stopping the BCG rather suddenly. With a doubled checked, cleared gun, remove the top cover (be sure to reseat the recoil assembly after...push the button), pull the trigger and hold it back and cycle the carrier. Does the carrier try to jump out of the rails as it cocks the hammer? Once the hammer is caught by the disconnector, keep the trigger back and rack it again a couple times. This disconnector will hold the hammer down further than the trigger, so the BCG should barely contact it. Any jumping? Release the trigger and rack again.
Other things to look at is make sure the grenade sight is all the way down. Look for any signs of rubbing on the piston/gas tube. Does the piston wobble a little (it should)?Look at the bolt and the BCG camming areas. Any signs of undue/uneven wear?