The nut is probably bottoming out before you get enough pressure on the prop to hold it. I picture that you have a smaller screw that holds the spinner onto the nut/adapter. If you are lucky a spacer or washer behind the nut will take up the slack, hold the prop, and still allow the spinner to fit tight onto the back plate. If not you might need a spacer between the thrust washer and back plate to get it tight.

rod