Very often that is caused by the take up spindle not taking up -- or not exerting enough take up torque. Remember that if this cassette player is auto-reversing, then both spindles must act as "take-up" at different times, depending on tape direction.

Lazy take up is usually caused by a worn, oxidized, or "dead" rubber belt. Replacing the belts may help. But also, some decks use rubber drive tires -- and these can either be cleaned and resurfaced or replaced.

Good sources of repair parts are:

Parts Express, and

MCM Electronics.
--
Don Foster (near Cape Cod, MA)