Sounds like your rifle is short stroking. Clean the gas system as best you can and try again.
Another possibility is that you didn't seat the magazines all the way in (operator error). With the bolt forward, insert a loaded magazine and slap up on the bottom with a little firmness to hear it "click" into place. Sometimes the mag retention button can press the mag against the magwell with enough force to keep it there without being properly seated.
Machine guns are awesome until you have to carry one.