If the second stage of the trigger is lost or the trigger does not return an adjustment of the 2nd stage spring plunger is required.
A trigger that will not return has a point to point engagement that does not allow the sear surfaces to slide by each other.
Each rifle, lower receiver assembly or trigger kit is supplied with a specialty .035" allen wrench in a Zip lock bag with trigger instructions.
Remove the lower receiver from the upper receiver. Cock the hammer back and put the safety selector on "SAFE". Use the .035" allen wrench to loosen the locking set screw for the tall threaded spring plunger in the left hand side of the safety selector. Put the safety selector on "FIRE". With a pair of needle nose pliers turn the spring plunger in clockwise until the second stage is felt and the trigger will reset itself. If the trigger does not return recock the hammer before readjusting spring plunger in some more until the second stage is felt and the trigger will reset itself when released. Put the safety selector back on "SAFE" and tighten the locking set screw.
ONLY Molybdenum grease or paste should be used to lube the sear surfaces. Any petroleum based oil or grease will cause the trigger to feel sticky and grabbing when the first stage is taken up. To clean and lube the sear surfaces remove the lower receiver from the upper receiver. With the hammer in the forward or fired position drift the hammer pivot pin out with a 1/8" drift punch. The hammer can be lifted out of the receiver and the sear surface cleaned. Use a cotton swab to clean the nose of the trigger in place in the lower receiver. Apply molybdenum paste to the sear surface of the hammer and just below it. Reinstall the hammer and pivot pin into the receiver. Be sure that the long doglegs of the hammer spring are on top of the trigger pivot pin and against the inside of the receiver to keep the trigger pivot pin from walking out.