Drive Problem?

I have recently purchased a secondhand rotary petrol mower, the mower has a drive, this was not working because the belt was off, i managed to get the belt back on from underneath (without removing any pieces). Now i find the rear wheels are rotating i.e the drive is working but without pulling the lever, so constantly in drive pulling the lever has no effect what so ever, the mower is a lawnpro!
Any ideas from anyone would be great, surely the drive is not shot as its constantly being driven? would this be something simple to rectify?? Thanks Martin.