Thanks for the ideas. I am going to try the sandpaper suggestion. I am using plastic spinner and plastic prop, which have very little friction.

I am quiet sure all the parts are on in the correct order.

It is falling off when we bump it with an electric starter and it fails to start. The engine stops and the prop flings off.

Would there be any harm in applying thread locker to the shaft? I was also thinking about installing a second prop nut on top of the first one?