Man, lots of things it could be.
Your problem is poor extraction.
High pressure makes for extraction with difficulty. Can be because of ammo, chamber or throat. Rough and out-of-shape chambers also extract with difficulty. Dirty chambers can also be the problem. Then thre is the classic problem of carbines being a bit too vigorous.
Examine your cases for evidence of high pressure and out of shape chambers. Check diameters of fired cases against those of your unfired ones. Check for signs of a rough chamber. Is the buffer correct? Both shorties and full size stocks should have buffers with weights that you can feel shifting when you shake it. Did you just buy it, or have you had a successful history before it started misbehaving?
Get back to us on that stuff, and we can help you get it sorted out.