Why can't I sync music iPod Classic 5th gen?

I recently decided to put some music on my old iPod Classic 5th Generation. However, when I opened iTunes, selected the song I wanted, and clicked "add to iPod" nothing happened. I even tried dragging the song both to the iPod name on the left sidebar, and to the iPod icon on the top of iTunes, to no avail. The iPod is recognized by iTunes and I tried updating it to the latest software (1.3) but that didn't change anything. It's nowhere close to full, so that's not the issue. I'd love to listen to music on this iPod, please help!