Pick up a bob sled so you can single load rounds in the chamber/lock the bolt down on the chambered rounds to first weed out if the problem is loading issue, or just a barrel/bolt issue.
If a feeding issue, then need to check the angle of attack out of the mag, up the feed lips, and them to make sure that the bolt is locking up the same way every time; either a first round cycle and when it live cycling as well.
If a barrel problem, then pull it to make sure its straight to start with, the chamber and bore is concentric to the side walls of the barrel, and maybe even lead lap the rifle as well. Also with the barrel off the rig, good time to not only polish the inside of the barrel extension lugs, but the bolt as well.
When the barrel is reinstalled, make sure that it free floating, the barrel nut is correctly tightened, and the gas tube is correctly indexed with the carrier key.
One last thing, and that you need to send a fouling shot down the barrel each time it's cleaned before starting your groups, and If you are allowing CLP to get into the chamber and bore, then it will take about 14 rounds until you blow the CLP/Teflon out of the bore so the barrel will settle in as well. Hence if you are going to run a AR for tight groups, then you want to run it without using much to any lube, much less CLP.