iTunes to iPhone cannot sync parts of the iTunes music library
How did you add the music to iTunes? Did you drag and drop it into the iTunes windows itself or did you manually add the music to the local folder?

If you manually added the music to the to the local folder then you need to "re-sync" your iTunes library so that the music appears in iTunes application.

Just choose "Add Folder to Library" and choose your iTunes music folder so that it "re-syncs" your library and ALSO adds all the new songs to your list as well.



You can add songs manually to your iTunes music folder without them appearing in iTunes if you navigate to the iTunes music folder itself on your computer and just drop the music in. iTunes tells you in the preferences:



But all of this is irrelevant until you tell us how you added the music to your computer???????

Hope this helps!