same issue here ... after adding new music files to my library. This seems to be weird, as it does not happen every time I add new tracks ...
I could get them on my iPod following these steps:
- connect my iPod
- Copy the new tracks to a different location (backup)
- Delete the new ones from the library and moving them to trash
- Import them again from the backup location using "Add folder to library"
- Close iTunes (leave iPod connected)
- Start iTunes again an sync
This did the trick (for today ... hope it will work next time again).