Drive belt slipping on Qualcast electric mower?

Hello, I have a Qualcast Classic Electric 30 mower. It has worked well for several years. But the drive belt started slipping off, despite tensioning it as directed. I have replaced the belt with the correct Qualcast spare (Ref: A57941) but it will not tension correctly - even with the adjustment at its' maximum position. When I run the mower, the belt slips off after a few minutes.
Any ideas, please?