For some reason, periodically one or two thumbs in a folder won't display a proper preview (usually the problem is they are not full size). Selecting/highlighting the thumb does nothing. I either have to purge the cache or open the file and resave it. REGENERATE PREVIEW should be a context (right-click) menu item.
It this feature already exists, please forgive me for not finding it yet. It still belongs in the context menu, though.