killall is a termal command that will quit the dock. Restarting the dock may fix the preview windows.
Open terminal and type
You'll see the dock disappear and reappear as OS X restarts it. Then try Mission Control again. Also try launching Mission Control using alternate keys including the MC icon, F3 or the mouse and see if you get any differences.
I guess I meant to say that it happened a while ago, I had rebooted, but didn't pay attention to the fact that the reboot cured it and the problem recurred. So, when you asked, I was referring to the fact that I rebooted recently (not when you suggested). In any event, thanks for your help