Some Preview Images are not shown in Finders File Preview

[britzke]

For some films the preview Image is shown in the Finders file list but not in the preview view, if you select the file in Finders Browsing Mode and also in Cover Flow Mode. For that films only one (I guess it ist the first) frame is shown in the preview. I tried to re-apply the image with metaz with no luck.
Is there information available on constraints for a preview image to be applied correctly?
Is there information available on constraints for a film to get a preview image get applied correctly?