The face of the barrel has an extractor cut that must be lined correctly.
The trunion has welded bolt guides so if there's something off axis, the carrier may not go back into battery correctly and give you intermittent fire.
The bolt gap must be set correctly for proper function. If not, function will be affected.
If you believe all the above are fine, there are things you can do to help.
Remove your trigger pack and put the bolt carrier assembly together with the stock/end cap. Hand cycle the parts and see if there are any friction points. This occurs when a roll flat is not straight (enough) and causes friction points which can drag and cause the malfunctions.
If the parts are smooth, insert a magazine (still without the trigger pack) with some dummy or live rounds and see if the carrier picks the round up from the magazine correctly and push the round into the chamber without stalling or getting caught up. Two things that can mess you up is the magazine well not being square and cause the mag to sit high or low. Most of the time a low magwell needs some trimming of the flashing. The second thing is the magazine paddle not being in spec or installing one that was modified and not giving you correct height.
Moving to the trigger pack, install the FCG back into the gun and examine the height of the pack and make sure it's not sitting too low. This can cause problems. See if the tail of the ejector is pivoting up and down to correct height and not rubbing.
The other issues can be the actual bolt head (since there are plenty non HK or RCM bolts) not being machined correctly as well as a worn recoil rod spring not giving the gun enough momentum to cycle correctly.