The description implies its the magazine, but worth investigating if the weapon is short-stroking.

Was the failure to feed a bolt-override (meaning the carrier was picking up the round, not the bolt face)?

Start by pulling the problem child mag apart, to give it a good CLP cleaning.  Even on new mags, they can have debris from when they where assembled, and such will cause all kinds of problems.  Also, take a good look at the mag follower skirt when CLP cleaning it, and if needed, break out a razor knife to clean up any molding excesses.  To add, here, make sure that the mag spring has been installed correctly.  I bring this up, since the spring coil that drops down from the bottom of the follower should be on the back side.


Next, when a mag is fully loaded, it should still allow the top round to be push down into the mag to almost allow another round to be loaded (top round should be able to be pushed down about 2/3 of the way to almost allow another round to be loaded).  Also, once you have the mag loaded, rap the back of the mag against your hand to seat all the rounds in to the rear.  During live fire, the recoil of the rifle is going to do this for you, but you need to make sure that round stack is all the way to rear before the mag is first inserted into the well.

Lastly, if you are loading a mag against a bolt locked home, use the palm of your hand to rap hard up against the bottom of it to make sure it seated in the mag well, and the mag catch is locked in place against the mag.  Hence even with a round seated all the way to back, and the mag correctly loaded to allow the  rounds to have the needed room to be pushed down on the mag body from the pressure on them against the bottom of the carrier, new spring pressure can be a bitch to over come with just trying to shove the mag up into the mag well to get it to lock against the mag catch.