Intermittent AVI files???

Hello...

I am using Quick time pro 7....Sometime when I save a my Tiff files (Aranged into a small movie) as AVI file, the thumbnail looks like a video and works in all software.... And other times (When saved as AVI) it looks like a piece of white paper with AVI wrote on there..... This latter one does not work at all???

I have checked all my settings but still can't save as AVI... Any help on this problem , many thanks Lance