Remove the washer behind the prop. There prop should be directly touching the backplate of the engine. If I had to venture a guess as to why it's coming off it would be because of this washer. The back of the prop and the backplate of the engine have a rough texture in order to keep the prop from slipping. What's happening is the washer that you have between the prop and and the backplate is allowing the prop to slip, in turn this lets the prop nut loosen and come off. Remove this small washer and I'll be willing to bet your problem will be solved.

Ken