Beko washing machine belt?

The drive belt of my Beko machine broke. I inadvertently had the wrong size belt installed which allowed the machine to work for a while whilst waiting for the correct belt. The incorrect belt dislodged itself on numerous occassion until the correct belt arrived.
The problem I have now is that the correct belt will not stay on the on the drive. It comes off the moment you turn the drum. What can I do to make it stay so that the machine can function