ORIGINAL: Crash-RCU
Are you using wood props, if so the wood is probably compressing under the prop washer causing the nut to become loose.
What is meant here is that at each flying session before you fire your engine up check your prop. Wood props compress slightly when tightened. that sets it up to slip when the engine starts. Then it just works the nut off like really fast. Moral of the story. Tighten your prop nut each day you go out to fly.
dennis