Why only Quick Look preview for SOME PowerPoint presentations?

[ProPedderKustom]

For some reason Quick Look is only showing me previews for certain of my PPT presentations. They are all saved the exact same way but only some of them will open and display the contents through Quick Look. Why is this and how can I change it? Thanks

[Pees330]

I have noticed this happening since I upgraded to Office 2008. Now when I download some powerpoints they get added extensions like .ppt.pot. Sometimes if I take the .pot off I can use it with Quick Look again. I think I read somewhere it's a bug that Microsoft is working on.