Use some after shave on a long cotton wool bud to physically clean the rollers, and the head, does it make a buzzing sound when on tape when playing, the heads can be become magnetised, a de magnetiser tape is only a few pounds, but I doubt it is this, the heads are so much harder than the cassette tape material that it needs some serious use to wear out physically.
A repair if needed replacing the head if it's that, is not as bad as you think.
I use Fullers, in Neddham Market, bob fuller owns the place this is the main repair place for all kenwood audio in uk, as well as doing pretty much all other makes repair wise.
Would be worth a ring 01728 746100.
tabetha