Looking at the picture, it looks to me that that spinner is way too small for the prop. What size spinner is it?

Try using at least a 2" spinner (if it's suitable for the plane). Most plastic spinners don't work well with APC props and need to have the slots widened due to the larger width of the blade at the hub.

If you can find one, try a SIG spinner, they have larger slots for the APC props.

When I changed from wood props to APC, I also changed to aluminium spinners as I was having the same problems as you. The spinner would come off, and it was a pain getting it lined up just right.

The only plastic spinner I've used with an APC was the spinner that came with my H9 Funtana, but I used it on a different plane with a smaller prop.