Sounds like you're under gassed. Gas block not properly aligned most common issue. Could be issue witb gas tube or port partially blocked as well.
If the port isn't spec from the shoulder or you aren't running a handguard retainer and don't have the gap for the retainer in there you'll be undergassed. If its so off the gas block isn't over the port at all you'll have a single shot. Sounds like the original block missed the port completely and the current one is only partially over it.
Loosen the gas block enough you can move it, but not so much it slides freely. Plug the chamber with a casing and blow air down the barrel with a compressor, canned air, or something. With a finger on the hand holding the casing feel the air coming out of the gas tube as you move the block slightly. Find the spot where you get the highest flow and tighten it back.
Maybe not the most scientific method, but it will help identify and rectify the issue.
(Helps to have a friend or a 3rd arm)