Try selecting the thumbnails (Shift+Click) or (Ctrl+A) to select all, then click on the top menu
Edit >> Update Thumbnail For Selected Items
If that doesn’t work close Elements and delete the thumbnail cache which sometime becomes corrupted. Navigate to:
Inside the MyCatalog folder (or whatever name you gave your catalog) you should find a CACHE file:
After deleting that file restart the elements Organizer and wait whilst it regenerates thumbnails.
N.B. The ProgramData folder is sometimes a system hidden file, in which case please follow these instructions before starting: