had the same problem with my first bushmaster (first AR). If had a fully loaded mag, 10, 20, 30 rounder, it wouldn't load the first few rounds, as it seemed to lack the push strength to push it out of the mag.
solution: NOT LUBED ENOUGH
and I had still lubed it slightly, you really have to coat the side rails at the base of the carrier (the only parts that have contact with the upper). I don't use CLP, cause I think that's not enough, use a specific gun lube like hoppes lubricant which is thicker and longer lasting than CLP. put a decent amount on both rails, and that should fix your problem, also make sure the gun is clean as possible, blast it clean with birchwood casey gun scrubber (anywhere, even wally-world has it). And lube properly.
try that.