Thanks for all the replies.
What eventually happens is the backplate of the spinner breaks away from the backplate on the prop shaft, so the prop/spinner assembly is turning while the engine is not. I didn't use to have this problem but once the engine broke in and the compression increased, it began.
Is there a different way to start an airplane engine than using a starter and spinner? I have a chicken stick and it doesn't work-there is so much compression that the prop cuts right through the chicken stick. So anyone know of a way to attach a starter to the prop itself without using a spinner?
Thanks!