Try some TALCUM POWDER on it as the slightly abrasive nature of this is often enough to promote the grip needed between pulley wheel(alt) and belt/s.
If it still does it change it, I found when I bought TWO belts foe my car that although they were from the same pattern supplier they were different widths only a small amount about 1.3mm if I remember right but enough to mean one belt loose one belt tight!!
tabetha