In your case, you may need to set up or update its settings for automatic syncing.
Select the iPod in iTunes. There is a row of buttons toward the top, starting with Summary. Click on Music next to Summary. This is the iPod's Music screen, where you tell iTunes how to sync songs to the iPod.
Check the box for Sync Music. If the iPod can hold your entire iTunes music library, you can choose to sync Entire music library. Otherwise, you can choose to sync Selected playlists, artists, albums, and genres, and then make your selections from the lists below.
When you click Apply, iTunes syncs your iPod based on those settings on the iPod's Music screen. Going forward, changes to your iTunes music library sync automatically, based on the settings on the iPod's Music screen.