i ran into this for the first time myself last week. no worries, it gave me something to do.
first off, tried different magazines. same shit. so i started going through pieces one by one. swapping out pieces resulted in the same shit. finally, i went with my original thought. and my thought was that it seemed like the magazine wasnt riding high enough in the weapon for it to pick up the next round. chambering the first round was fine, but when fired, nada. so i swapped out the entire lower with a different rifle. volia! the lower reciever was the culprit. slapped it on the other upper, it ran like a song. so i ended up just swapping the lowers on the two rifles.
after looking at the obvious stuff, and before you start digging into the no-so-easy stuff...swap that lower out and see what happens.