Live Icons on Task Bar Aren't Always Live

Hello, I've been having a strange problem since day one of using Vista
Ultimate...when I hover over an item on the task bar it presents a live icon.
If I haven't uses a program/window recently, when I hover over it, instead of
showing me the live preview, it shows a blank white window image with a small
program icon in the middle. It I click on it to restore the window, then
minimize it again, the preview icon shows a live image...for a while. If I
don't open the window for a while, it goes back to the empty window/icon
preview. How do I get this to work properly? is there a setting that needs to
be changed?

Thanks

[lagunitas]

I have the same problem - it happens with powerpoint. If more than one is open, I get the same preview. It's a window with a blank white background and little picture of a program icon in the middle.

Anybody have suggestions? Thanks