I agree with Stick and others, you aren't tightening the nut enough. I CAN and do get them tight wnough with a 4 way glow plug wrench by the way. I have 5 planes flying currently, three with four strokes, and have had hundreds of flights over the past eight years. I've had props come off twice. (Not counting your's David. )

Four strokes that throw props are usually set too lean.
Dennis-