We had a similar situation back in September where the thumbnails would not show for some "related data" in Open Data. It turned out that one of the Open Data updates had inadvertently allowed "ghost items" to haunt our site... whatever that means. Apparently, "ghost item" thumbnails don't show. The fix was a bit complicated and this may not be the same issue, so I'd suggest that you enter a support request with Esri.
Happy Friday.
