Try selecting (highlight) the thumbnails and from the menu click:
Edit >> Update Thumbnail for Selected Items
If Update Thumbnail doesn't help, try renaming the following file - thumbs.5.cache to say thumbs.5.cache.old. This file can be found in you catalog folder whose location you can collect from Help >> System Info in Organizer.
It might take a while if the no. of files in the catalog is huge, to regenerate image thumbnails.