Is there any way to force Windows 7 to refresh the taskbar previews?

I frequently find that some of the window previews in the Windows 7 taskbar will be lost, especially after a window has been minimized or hidden for a considerable period of time. Instead of the normal preview of the window I just see a gray box with the program icon in it.

Example:

This is a pain when I have a dozen Firefox windows open and can no longer quickly distinguish windows in the preview. Is there anything I can do to alleviate this? Any way to tell windows to keep around the previews longer or some key combo that will refresh all of the previews?

Try this:

1. Click on the Start Orb > right-click Computer > click Properties > click Advanced system settings
2. Under Performance click Settings > Check if Save taskbar thumbnail previews option is selected or not.
3. If not selected, select it.

That should save the thumbnail previews and you will not get the grey application icon preview.