Like it was said, WordPress only generate thumbnails in the new sizes of the new media uploaded. The old images’ thumbs doesn’t get generated automatically.
For this situation, I strongly recommend the following plugin:
http://wordpress.org/plugins/force-regenerate-thumbnails/
It is pretty simple and unobtrusive. It just adds a new item on the wp-admin menu for you to regenerate every images and its thumbs, every time you insert a new image size on your theme, all in one click.