I bought a Blackthorn 20" 308 upper for my DPMS. Put my BCG in and installed on my DPMS lower. Bolt seats fine. Fired a round and bolt didnt eject and the BCG wouldnt pull back. Had to strike but on ground to eject catridge. Bolt works fine until you load a round and fire. Then it is hell to get it out. Headspace?? What can someone tell me?