my drive belt keeps coming off Hotpoint aquaris wml 520?

Hotpoint Aquaris WML 520.
Hi, our drive belt keeps coming off, I have tried to refit it but as soon as it spins it "pings" off again.
I have put it on the pulley correctly but it does not seem to sit in the middle of the drum wheel so it seems to slip either off the front of the drum wheel or off the back.
The tension seems correct.