Lets start from the beginning, The rifle is fully ejecting, and locking back on the last round. By fully ejecting, you do mean that the case is being thrown a good 10' or better. If this the not the case, the barrel gas passage may be blocked. Start with the gas tube flow(use a tube to test), then move back to the key to confirm that it is tight against the carrier.
The signs that your rifle is showing is that it is short stroking, and somewhere you have blockage, or a leak.
Pull the gas block and check the alignment or the two, both set back and rotation. On one of my rigs, I had to keep the block off the shoulder a few mm to keep the ports aligned. It also helps to use a pencil line on the barrel shoulder(gas port index) to blind align the two.
Let use know after you have flow tested the system. Use a piece of tubing if needed.