If the gun is under gassed, it will not drive the op rod far enough back to catch on the trigger sear, but far enough back to extract, eject and chamber a fresh cartridge. Either the gun is dirty, or the gas piston/gas cylinder fit has excessive clearance. Turning the gas adjustment to the next larger port, will normally correct the issue. But, do check the clearance between the gas piston and the gas cylinder. They are often worn, creating excess clearance.