First, try different ammo. Q3131A is my test load, always.
Second... your rifle is short stroking. This can cause the no-lockback-last round failure, and feed jams of all types. This is typically binding, or gas issues. Since you have a custome gas block, I'd start there. Make sure it is aligned with the gas port on the barrel, and that the gas tube is properly installed and pinned. Next, check the bolt carrier. With it removed, does the bolt pull forward and back without much difficulty? Inspect the gas key. It is loose at all? Check the screws on top to make sure.
Binding..... well, I pull the carrier a little more than halfway out of the upper.... then point rifle down at a 45 degree angle. Carrier should slide into place and bolt should lock or almost lock without much extra pressure.
Also, check binding on the buffer tube. When you manually charge it.... is it smooth, or does it bind at all? Pull it all the way back, and slowly "ride" the charging handle all the way up. Does it hang up/bind anywhere along the way? Doing this, does the carrier go all the way forward.... and almost lock, taking only 1 or 2 hits on the forward assist to lock?