I purchased a MAK 90 from a guy I worked with a while back and was told that it was never fired. After purchasing it, I had to install the trigger and oil it up before I took it out to test fire it. Upon firing it, I noticed that it would not fire in semi auto. When fired, it will reload the next bullet in the magazine but does not push the hammer back down. Would anybody be able to help me figure out why this is happening? I am not a gunsmith but I have a good knowledge of how things work and the only thing that I can think of that the vent holes in the gas tube are not allowing enough gas pressure through the tube to push the piston all the way back and knock the hammer back down. Any information would be greatly appreciated!