This kind of problem is frequently caused by the use of a plastic spinner. If the spinner backplate is plastic, it is more likely to result in the prop-nut coming loose than if you use a spinner that is either (a) all aluminum, or (b) is plastic at the front, but has a metal backplate.

Additionally, make sure you snug the prop-nut sufficiently, and always re-check it after any landing in which you nick the prop.

Gordon