because the wheel weight is not the same when full and empty, for sure its a "drive" problem, could be a clutch wheel assembly
one thing you could do is empty a cassete just keep the contour frame, and sometimes you see a gear going from the supply wheel to the takup wheel
need to find the service manual and see the drive mecanism parts
hitachi had tons of theses problems, and you had to chane a mechanical assembly to get things working again
some clucth where easily repairable, you could dissasemble them, use a tooth brush on the felt disk, and slightly over-expand the spring to give it more push force