Commercial tape head cleaners only work for a limited time.
Over constant playing, the pinch wheel rubs oxide off the cassettes and it builds up.
If possible, take your tape player out, and take the cover off. You'll see the pinch wheel, next to a silver pin.
Get a cotton bud and dip it in white spirit, and rub a section of the wheel - you'll need to hold it still, so it's a bit fiddly. You will see a load of brown/black gunk on the bud - use the other end, and more, until the bud is clean. Rotate the wheel a fraction to expose the next section, and repeat.
When the wheel is clean, do the same for the silver capstan - making sure you run the bud up and down from top to bottom.
When all the gunk is gone, rub a clean soaked bud over the pay head - the little rectangular piece that the tape runs over - more gunk, so repeat as above.
Once everything is clean, reassemble and refit, and you should have nice clear sound.
Repeat every couple of months to prevent the years of built-up gunk you deal with the first time.