Within the trigger group I would look at:
Hammer Hooks altered or damaged.
Sear altered or damaged.
Sear pin damaged or broken.
Trigger lugs altered or damaged.
Trigger pivot pin altered or damaged.
The next steps would be under short cycle but I would remove the trigger group and manually cycle it and make sure it cocks and works. The safety won't engage unless the hammer is back and cocked. Also, if you can manually cycle the rifle using the op rod and lock the hammer back I would focus on short stroking as an issue. Short recoil can have any of a dozen or more causes. Some have been already mentioned.
Gas piston undersize.
Gas piston out of round, deformed, or damaged.
Gas cylinder oversize in power stroke area.
Gas cylinder out of round.
Gas cylinder deformed or damaged (drags, binds, stops gas piston).
Gas port in barrel plugged or restricted (carbon or foreign material in gas port).
Gas port in barrel restricted (gas port indented or peened).
Gas port diameter undersize (a problem in some commercial barrels).
Barrel O.D. undersize in gas port area (power diameter undersize).
Gas cylinder loose on barrel (gas cylinder mounting ring I.D. oversize).
Gas cylinder lock screw loose in cylinder.
Valve in gas cylinder lock screw doesn't close (valve warped or carbon build up).
Valve in gas cylinder lock screw leaks (valve cracked or damaged).
Operating rod binds (lower band/handguard assembly fit problem).
Operating rod binds (gas cylinder misalignment problem)>
Operating rod binds (op rod bend/fit problem).
Operating rod spring kinked or deformed (drags, binds, stops operating rod).
Operating rod spring rust stuck inside operating rod spring passage.
Wrong/too long operating rod spring (spring coil binds).
Follower rod body bent, altered, or damaged.
Follower arm twisted or bent.
Bolt assembly binds in receiver (damaged, distorted receiver).
Bolt assembly binds in receiver (warped and/or off-centerline rewelded receiver).
Bolt operating rod cam lug/operating rod cam surface bind.
All of the above taken from
The US .30 Caliber Gas Operated Service Rifles Shop Manual by Jerry Kuhnhausen.
Ron