I don't think their is anything wrong with your rifle, the 223 ammo powder might be optimized for longer barrels and not have optimum gas at the gas port for the shorter barrel, maybe that combined with a new rifle that is not broken in yet.
Another thing, did you grease/lube the recoil spring in the buttstock? I have a New Frontier lower on one of my mid length carbines, and I noticed at first that ejection was not as brisk as on my aluminum lowers, it was just barely trickling out, and this was shooting reasonably hot handloads. I did grease mine well, and it improved some. Just lube everything well, and shoot it a lot, might just need a break in period. My one Bushmaster lower has about 15,000 round through it, and it is as slick as a goose.
I don't have time for busy people