mine was the same, but do the following
make sure the barrel ports are not plugged
get a kings armory tappett
lock the bolt back and leave it for a couple of weeks
polish the rails inside
if this doesnt work, then you may need a gunfixer plug
Uh, why not just skip all the above shit, (except checking the gas ports, obviously), and just buy a Gunfixr's gas plug? It's superior to the factory plug in every way, allowing you to have both more gas settings and to hand-adjust those settings at any time, without fucking around with a seperate tool.
Gunfixr's improved