First check to see if the bolt carrier key screws are loose by trying to tighten them with a 9/64 allen wrench. If the key screws were loose they need to be removed so the key can be properly reinstalled.
To check to see if you have a leak at the bolt carrier key causing short stroking pull the bolt carrier assembly from the upper receiver. Push the bolt into the carrier and hold it there while performing this test. Spray a light oil into the key. Blow a couple of pounds of compressed air into the key. If you do not have compressed air place a rubber tube onto the key and blow into it. If there is bubbling between the key and the carrier it has a gas leak.
To correct the gas leak remove one of the screws and put it back in. Remove the second screw then go back and remove the first screw. This gets the screws past the staking so that they will not be stuck in the key. Clean the key and the carrier with brake cleaner to remove any carbon build up. Put a drop of oil on the bottom of the key and wipe it off so that there is just a thin film left on it. Install the key by torquing the screws at 45 INCH pounds. The key can be restaked with a dull prick punch or cold chisel so that some of the metal of the key is pushed against the screw heads to keep them from getting loose.