iPod Not all of my purchased music has been synced to my iPod

I'm helping my mom with a problem she is having with her 4th gen iPod connecting to iTunes 11 under Windows 7. She has purchased a few albums in iTunes, and all but one of the albums she purchased through iTunes has synced to her iPod. The strange thing is, when I browse her iPod through iTunes, it shows that album unde the "on this iPod" section, it just doesn't show up in the iPod music app. Any ideas where to start troubleshooting this?

thanks!