Frank nailed. Take the gas tube off and shake it. If the piston doesnt move freely inside then its probably gummed up with cosmoline. You can also check the gas valve, when the grenade sight is in the stowed (flat) position the gas valve knob should 'lock' over the top of the sight rail. That is...
Well they arent near as nice as some of the others that have me feeling strangely inadequate but heres my contribution.
Thinking of putting a Tapco set-up on the SKS, cant make up my mind. Some days I like it the way it is, other days I want to dress it up.