One common problem is alignment of the gas cylinder with the gas port in the barrel. On a number occasions I have had customers who had the same problem it turned out to be that the gas cylinder tighten too far back on to the barrel with the gas cylinder partially blocking the gas port thus not providing enough gas pressure to fully cycle the rifle. I use a common bent paperclip to ensure that the gas cylinder is properly aligned with the gas port before tightening the gas lock to the barrel and gas cylinder.