I've been trying to create a custom image size for featured images for the past hour. All my code was correct, but for some reason the images weren't working. Finally, I sought help from a coworker and they said they've had this issue in the past too.
It was suggested I download a plugin to regenerate the image sizes (http://wordpress.org/plugins/regenerate-thumbnails/). Soon as I followed the steps within this plugin, it was like magic - finally the custom image size was working.
I just wanted to share since I spent so long and there's nothing written in the Codex (http://codex.wordpress.org/Function_Reference/add_image_size) to indicate that this needs to be done.