It sounds like short stroking, just place 1 cartridge in the mag, chamber and fire.
Does the carrier lock back on the mag catch after firing? If not then it is short stroking and not going back far enough to catch the rim of the next round.
Remington UMC and Federal eagle are known to be weak, try Winchester White box (XM193) those are full power loads.
There are several things that could cause short stroking, sometimes it just needs to be lubed well but keep the bore dry(no excess oil) If it has an H2 buffer you may want to go to a lighter standard carbine buffer. If the mags are stuffed full there could be too much pressure on the carrier as it slides over, load the mags to the top and remove one round as standard practice. The gas block may not be in line with the port in the barrel and as a last resort the port in the barrel may need to be enlarged, it's easy to do but not really reversible so that would not be the first choice when looking for a fix.
Hi perf barrels and bolts
8 twist, 5R, Melonite, Wylde chamber