Drive Belt comes off usually when I chnge directions?

Dont know the model, but from what you say, engaging clutch, must move the parts away from each other, thats why it is easy to get it on then. As it comes off, that is usually down to a slight misalignment, both units need to run square to each other, other wise belt will be driven off, which it sounds as if that is happening, some part of the clutch mechanism, which ever part it moves, is sticking, or catching on something. ie. if motor moves when using clutch, possibly slow to move at bottom end, so belt will come off, etc. Check as to what moves, clean and if needed grease runners etc, until you have a good look you will not see why, but hopefully I have pointed you in correct direction.

SD
December 2011