Hi
First off you need to clean the tape head with isopropyl alcohol as over a long time the tapes leave a residue on the heads you should also clean the rollers and guides. at the same time.
If this does not make a difference then it could be that the tape head has gone out of alignment to the tape,check the rollers and guides to make sure the tape is running where it should.
If the tape is running correctly then most machines have adjustable heads ,this may not be so easily done nowadays as you used to be able to buy alignment tapes that you listened to or watched an oscilloscope trace as you aligned the heads.