Make sure the carrier key isn't loose, next rings, next make sure the gas tube is straight within the receiver and isn't draggin on the carrier key which could make it leak also and not feed properly. Your gas blocks gas port may not be lining up properly with the barrels gas port. If everything else checks out you may need to remove the gas block and mic how far the gas port is from the end of the gas block and then the barrels gas port from the machined notch in the barrel (where the gas block rests). I know from experience that the PRI gas block on a DPMS barrel on my wifes M4gery had to be moved .071" forward to get enough gas to operate properly, funny how two different companies equipment has tolerances that far apart. Before moving it, it had only been getting about 5% of the gas it was supposed to and wouldn't even move the bolt far enough rearward for ejection. Runs like a top now with an H buffer even on weak American Eagle and Natec ammo.
Edited to add. You may need to adjust your mag catch if its not picking up the next rounds also, or your bolt catch is out of spec. Check that gas tube within the reciever, that to me sounds like the culprit of the sticky bolt.