Thumbnails on desktop not showing

Binary package hint: nautilus

I am using gutsy. I have four image files on my desktop (jpeg and png). Sometimes Nautilus shows these as thumbnails, as it should. Sometimes they revert to the icon which usually means "thumbnail generation in progress" -- i.e. a document icon with an orange "timer" in the bottom corner.

If I do not click on the icon, the thumbnail will not return: the icons remain as the "in progress" icons indefinitely. In standard Nautilus folder windows, this is not a problem: although fetching the thumbnails is sometimes slow, they always arrive. The problem seems to be specific to the display of the desktop.

Even when the thumbnails are displayed for image files on the desktop, they sometimes revert to the "in progress" icon; and I don't know what triggers this.