Inactive for over a decade
Sounds like your having gas leakage between the gas port barrel block and the tube that fits between it and the rear sight support assembly. Have had several Yugo SKS's with this problem which can be fixed fairly easily. Some people just use the right size o-ring but life expectancy is rather low with this method. I take a piece of wire and wind it around drill bit then cut off and make a ring similar to a piston ring that slips over the step on the tube end. It will fit into the recess of the barrel gas block. You have to experiment to find the right diameter of wire. Usually a straightened paper clip will work. This seals the extra clearance space enough so the full power of the bled off gas works the piston rod cycling the action.