If you are using a propeller made from a plastic material and a spinner it will happen often.You need to put a disk of sandpaper glued back to back between the spinner and the engine and then some between the prop and the spinner. The prblem now is that there isn't enough friction between them no matter how much you tigten the prop nut. Also, try using a box end wrench. You will save the prop nut that way when you apply the torque. If you switch to a wooden prop you will be doing yourself a favor. Dan.