I suspect that the take-up loading gear is damaged. This is a very common fault in the mechanism this model uses.
The result is that the supply side loading gear (left side) does not complete full travel during loading. So there is low tape tension around the cylinder.
Repair cost est. £50 'ish.
(To confirm the fault,.. on viewing the empty mechanism from above, (power disconnected), you will likely see two 'tiny' specks of white plastic (severed gear teeth) stuck to the grease on a toothed metal rail just infront of the cylinder near the centre of the topside mechanism.)
There are other possibilities, but the above is 'very' common.
Cobweb
July 2007