I did have some issues when I was setting everything up, and I just kept cleaning up tags, finding more art. I removed some of the files that don't clear out when you try to start a new file, and kept plugging away at the tags and things started getting better and better.
I would try one thing before you give up. I would uninstall the app, make sure the entire directory is gone, verify the folder structure is correct, verify all the tags look good in Mp3tag, take another look at the covers, then reinstall Freebox and start over. Can't hurt.