Does this happen with all your 20rnd mags? or just some? Are they marked and numbered, so you can segregate them?

Is the round actually making it into the chamber, or is it getting jammed up in the locking seat area?

Have you tried a full mag while applying pressure, or not, to the magazine body either from bottom, front or rear?

Are the magazines new, or worn?

How many rounds on the rifle? What is the condition of the action spring?

Does the gun have M4 feed ramps? Do the ramps on the barrel match those on the receiver?

Does this happen with full 30rd mags?

With a bit more information, I can probably help you work out whats going on, or you can just pop into the shop and we'll sort it out.