From your photo I'm guessing I'm seeing the thread ridge which doesn't seem high enough to push the pin down.
But another thought comes to mind.. Does the pin have a nice sharp 90deg back to top angle? (check to see if it's rounded over)
The grooves mentioned seem like a good idea, even better would be a knife edge at the top on the button that would "cut in" to the spinner face on contact and thus lock the button in place. Or possibly a groove in the back of the button with a raised lip on the spinner face.
Or of course say the heck with it and get the safety wire out...
Definately take up David on the billet spinners, they're really nice pieces.