When I had a Makarov I found the same thing ... the last one or two lead bals would effectively jam.
If you get one of the jammed balls and inspect it, you will see a 'flat with a slight depression' on it ... I concluded this was caused by the balls beneath the damaged one bouncing on it as part of the rebound effect of the gas. The gas would fire the ball in the breech but also exert some force on the balls remaining in the magazine.
I found that all but the last 2 or so balls fired OK from a full magazine. Generally, only loading 10 balls meant that all would fire Ok. I got over the issue by loading one or two steel BBs into the magazine first. Remembering to 'stop' and not shoot the steel balls was no real problem. I tried to find copper coated lead balls but failed. I thought they might solve the problem as they had a harder coating and may not have deformed.
Cheers, Phil