Auto Trace Not Previewing?

Hi-

I'm trying to trace a video layer, but for some reason when I check the preview dialog box, the mask flickers on and off, but dont stay visible.

Specs:

Max OS X 10.7.5

3.06 GHz Intel Core i3

8GB 1067 MHz DDR3

AE CC Version 12.0.0.404

Thanks for any help.