My first thought is to get different magazines that are reliable. Mags are one of the main causes of failures. So get some Pmags, good GI mags, Lancers, or whatever you can get your hands on. Being limited to one mag is hard to tell because you can't be sure if its the mag or the gun.
Once you figure out its not the mag, then you can troubleshoot your AR. With my builds I usually try to break them in with hotter 5.56 ammo only for the first hundred rounds or so. Nothing wrong with 223, but personally I like to use 5.56 as much as I can.
My last build was a 16-inch Spikes Tactical carbine and I did experience a couple of Failure to fires in the first 40 rounds. Since this is a build, we have to make sure that all parts fit and are installed properly. Things like gas blocks loosening or springs not installed correctly can give you issues. After about 100rds through my Spikes AR, it seems to have smoothen out and it shoots great now.
Good luck and keep us posted. Congrats on your build and it looks like you have a nice rifle there.