Also you said the ammo is 223/5.56.. which is it.. they are almost interchangable but 223 offers less pressure than true 5.56 mm and will not fully cycle some tighter guns.
Especially those that have been "improved" with a full-auto BCG and "H" marked buffer.
From your descritpion it sounds like it might be similiar to a problem I had one a used gun.
It wouldnt cycle properly on 223 only on 5.56.
It had a full auto BCG (adds weight) , a H Buffer, and a rifle lenght spring(!) in it.
Putting in a carbine lenght spring and a lighter buffer may have fixed it ,but i have not shot 223 in it for a while