No Preview - sort of. :|

[Bald Eagle]

I've noticed that some of my VISIO files are fully previewable in the side panel (Windows 7), whereas others say "This file can't be previewed".

Except that Win 7 _IS_ previewing it in the tiny little thumbnail view at the bottom of the directory.
I find this to be strange behavior.   Is there any explanation or fix for this?
Just wondering.


Thanks!