If possible, use some q-tips with at least 91% isopropyl alcohol to clean the head, capstan, and pressure roller.
If you can't reach them, try to find a decent cassette cleaner. They have a regular cassette shell, but brushes and such inside to do the job. I have not used one of these since 1985, but a couple of the decent ones were made by Discwasher and Allsop.
Also, the azimuth angle on the tape head may be out of whack.