If your using the orginal mag make sure the pusher is in the center of the mag if its off to one side the round that is on the other side has more clearance to pop up free and its not held in waiting to be loaded mode. If all the other rounds feed fine its the only thing i could think of thats wrong the other bullets hold the rounds even till the last few come up then the pusher has control and influences of whats going on next with the last rounds by pushing the rounds to one side allowing the one to pop up free.
BigBill
I did have a one orginal mag that the pusher was off to one side and the orginal mags are different than any other with that rod/lever system but when its right its dead on. I haven't had too much trouble with sks's just this one mag problem actually it was the orginal fixed replacement 20rd mag "chinese star" I could see the center was right away it was that bad.