Knowing what ammunition you were running would help. Was it commercially made by a reliable firm, like Winchester White Box bulk, or was it "something else"? The amount of power needed to cycle the standard 9mm AR isn't huge, but if you have am extra heavy buffer and/or spring, you might just be slowing things down a little too much, especially once the gun gets dirty. Your statement that the first 20 rounds went without problems makes me think that it got dirty enough to slow down extraction enough that the bolt didn't go quite far enough back to fully cock the hammer.
A blowback 9mm AR carbine is a filthy beast, and it gets filthier every time you fire it. "Tuning" the system with a stiff spring, beefy buffer or other tweaks, especially without having had any real problems without these tweaks, just confuses everything. Use a stock buffer and spring and see if you get these stoppages.