A plugin for updating your thumbnails whenever a new thumbnail size is added with add_image_size()
Possibly permission problems. If you're like me, you probably have a two-year-old blog and have switched hosts at least three times in that last year. What does that have to do with permissions? Well, some of your older uploads—those dating prior to your last webhost change—either A) don't have the folders and files set to 777 and 666 (respectively) or B) Owner:group isn't set to www-data:www-data. The first problem is relatively easy to deal with using any good FTP program. The second problem requires that your web-host allows shell-access to their servers. If not, you'll have to contact them to get the groups changed. Finally, I've run into some issues with the oldest files on my server. It didn't matter if I changed permissions or owners on the directory or files, I still could not get thumbnails to generate.