Board Thread:Support Requests - Designing Your Wikia/@comment-1807002-20150904042558/@comment-4731443-20150904154025

Category thumbnails have a few restrictions for the images they use. Kirkburn decribes the process in detail here: User_blog:Kirkburn/How_images_are_picked_to_represent_articles

The short answer in your case seems to be that the ones that don't show up are too small (128x128 while they need to be 130x115 or more).