Check your settings. If you have the newest version of iTunes, plug your iPod in and when the update page comes up check the different sections. Under "Music" make sure you have "sync music" checked, and do you have "all songs and playlists" selected or "selected playlists" selected? Also, on that main update page that comes up, make sure you uncheck the option for "only sync checked items" and that you uncheck "manually manage music and videos"
If you don't have the "sync music" or "sync video" or whatever, it won't automatically update when you add music.
Sometimes mine gets glitchy and I have to disconnect my iPod and plug it back in, or restart my computer and bring iTunes back up again. But it sounds like you might not have it set up to automatically transfer your library.