Did you ever say what ammo you're using? Some guns won't run worth a damn with Russian steel case ammo. It's not loaded the same as XM-193 or M855 spec ammo, and the different pressure curve from the powder the Russians use isn't well balanced with the AR gas system. Plus the cases don't expand the same as brass, so you get a little more soot making its way into the chamber. It just causes problems with some guns.
You also could have the wrong weight buffer in a gun built or worked on by someone who didn't read enough before buying parts. A couple of the factory built AR's I've had were marginal with steel case Russian ammo and even noticeably less enthusiastic with civilian .223 ammo due to gas port / buffer weight combination.