FERM belt slipping?

The belts on my FERM FBDS350 have started to slip and stall when sanding. Motor keeps going but the belts stop. Even new belts. I can't see any way of adjusting the pulley, other than the centering adjustment.
Richard Barton, September 2020


done
is it the sander belt that is slip[ping? if so has the rubber drive belt inside worn?

Rupert, October 2021