tighten up your recoil buffer assembly, when its loose you often can't get it to fire.
Take the trigger out and re-install it. That worked for mine, just a burr or something.
Also if you are using reloads, run them through a wilson or simular case gage. Excessive headspace on the cases will cause both problems.
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071