hi, their could be a couple of reasons why this is happening, either your not putting it on so that it sits square on the main pully and the motor or the bearings have gone in the drum in which case you need to call a repair man, put the belt around the motor first then onto the main drum pully then turn the drum untill the belt is on turn it at least 2/3 whole turns before turning on the motor make sure the belt is sitting in the same place as the old belt you should be able to see a shinny part of the spindle where the old belt was it should stay on now check the tension on the belt it should have a little slack in it. hope this helps
chris
May 2008