No, Bolt is not locked back. It occurs during the cycling Sequence:
3) - When you fire, the chambered round is ejected, the bolt carrier moves back and resets the trigger
, a round gets ejected from the tube onto the lifter...
-> Bolt is not going forward but is stuck in the rearward postition, Round is on the Carrier, but Carrier does not come up. Have to give it or the bolt a tip.
... the bolt comes back forward, triggers the lifter, grabs the next shell and chambers it