As the bolt comes forward it strips a new round from the mag. The case slips under the extractor as it slides up the breech face. If the extractor is nicked or tight, the round cannot slip under it and will stick. Manually cycle it slowly and see where it hangs.
Also, are you getting 'trigger slap' when you fire? check your FCG.I ran it as I got it and I installed a buffer at the back of the spring and still nothing