The tape tends to stick to itself because of the tight winding. I once rescued 20 or so tapes that had been under water... You may make progress by pulling all the tape out by hand - carefully into a box - and then winding it back on by hand - I made a Lego winder to speed up the process - then run the tape on high speed wind backwards and forwards a couple of times.
Magnetic tapes on computer systems used to suffer this problem - the wisdom was to wind them back and forth every few months to keep them in good order.
