I recently assembled two ARs. Both kits from Model1 and both receivers are DPMS. One works perfect while the other is constantly jamming. The rounds are not ejecting from the chamber, they stick in the chamber and I need to use a cleaning rod to knock them out. There is nothing wrong with the extractor. I cleaned the chamber/bore and lubed the bolt, all the basic stuff. I'm thinking that the upper was not headspaced correctly but not sure as my AR experience is limited to carrying an M4 for 3 yrs. Whenever I had a problem like this I gave it to the Armory. Any suggestions??