How can I fix my Simpson Maxidry dryer rear drive belt?

How can I fix my Simpson dryer to stop the rear drive belt coming off. I have removed the rear belt cover to expose the belt driving the tumble mechanism. The original belt kept coming off so we replaced it with a new belt. This is also continuing to come off. We are at the point where we do not rehang the dryer so we have immediate access to keep putting the belt back on. Is there a fix for this by some other adjustment or should we ditch it for a new dryer.

Thanks Chris