Are you sure the files are mp3 files? The Clip+ does not support AAC format, unless you install the free alternative firmware Rockbox. Rockbox offers many additional features and settings, however it is more complex than the firmware the player comes with. Rather than installing Rockbox, you could use itunes to convert the files to mp3 files, then copy the mp3 files to your player. Doing this though reduces the sound quality, so I suggest doing this with copies of your files, and keep copies of the original files elsewhere.
If your files are indeed mp3 files, then make sure the format for the tags is ID3V2.3 ISO8859-1. Itunes uses a different tag format as its default.
