Sometimes the gas port in the barrel is not exactly where it should be. Occasionally getting the gas block properly aligned with the port in the barrel means you can't have the gas block square against the barrel shoulder, or even have the gas block precisely at 12 o'clock.
Pull the gas tube out, loosen the set screws on the gas block, and use a flashlight to check that the holes are mating up. Or pull the gas block and measure everything with calipers.
That said...if all your buffers and tubes and springs are correct...
...and your gas block hole is perfectly lined up with the gas port on the barrel...
And the gun still doesn't cycle...
Then the gas port in the barrel isn't big enough. It is pretty easy to drill them out larger. Just stick an old cleaning rod into the barrel to protect the barrel and run a slightly larger drill bit in, the smallest step up you can, might be 5 or 10 thousandths. The more barrel you have in front of the gas port, the more this increase in gas port size will be magnified.
