External video playback plays on main monitor but not secondary monitor

Hello everyone. I edit on 2-30" Apple Cinema Displays. When I make a cut and want to view it on a larger display I select video playback-Digital Cinema Desktop Preview-Raw. It's my understanding that I should be able to view the playback in full screen on my secondary monitor. However, the video doesn't playback. The image on the screen is just black. If I select a similar option but Digital Cinema Desktop preview-Main then the video playback plays on my main monitor. I trying to figure out how I can fix this so I can preview the footage on my secondary monitor (mainly for convenience) Thank you ALL for your input.

Erv