Make sure if you want to sync all your music that under the Music tab in your iPod, that you check 'Sync Music' and 'Entire music library'.
If you only want a selected amount of your music then check 'Selected playlists, artists, albums, and genres' and lists will appear with all your music content and you can easily select what music you want.
If you've already done this, have you made sure your device has enough room and you do not see this:
If you are over capacity, then you will need to clean up your device by removing any other apps or videos you don't need which take up a lot of room. It also helps to just select particular music that you listen most often to to sync rather than syncing your entire music library.
Also make sure your music is actually in your library and you don't see exclamation points next to all your songs.
If you have this problem, read this articleon how to fix this issue. This is only possible to fix if you have the original audio files somewhere on your computer or an external hardrive or disk somewhere else.
I hope some of this helped 🙂
If not, try reinstalling iTunes or restoring your iPod. Annoyances like these often occur and can unfortunately only be fixed by restoring and reinstalling.
Good Luck.