I bought a Yugo SKS and took it out to fire. I got 1 or 2 shots that cycled fine. The next 10 had to be manually cycled to eject them. I took it home and cleaned it thoroughly, including the piston and gas tube with crocus cloth. Took it to the range again and not a single time out of 50 shots did it eject the case!! What gives?? My gas port in the barrel is not blocked off and the piston rides nicely in the tube. Any suggestions would be appreciated.