That's odd. I just made a playlist of all my MP3s (all 12 that are not podcasts) and they all synced just fine. Played too.
My guess is it's probably a tag issue. Try right clicking on one of these files and choose Convert IDs tags. Use version 2.3. Then sync. I think that will fix it.
Have you backed up your iTunes Library this week? If not, you are risking losing your music and playlists....