I've had this problem a couple of times. One time it was because the belt that turns one of the spools that pull the tape had broken. Consequently, one of the spools (the one that the tape was winding off of) was turning, but the one that was supposed to take it up wasn't.

The other time it had something to do with to motor thingie that turns the gears that pull the belts. Again, same result as the broken belt.

In both instances, the repairmen said I would be better off buying a new tape deck because fixing it would cost almost as much as buying a new one (they were both pretty inexpensive to begin with). I don't know if they were telling me the truth or not. Sounds like you either need an expert, or a new tape machine.

Carol