If a song will play in iTunes, it should play on your iPod as well. Generally, the only reason a song won't play (assuming it will play in iTunes) is if there is some sort of DRM issue.
You might find that DEAUTHORIZING your computer in iTunes and then REAUTHORIZING it with your iTunes username/password will fix it. I believe you're allowed to deauthorize your computer whenever you want, and you can deauthorize all 5 accounts (the max you get with a purchased song) once per year.
They did recall a few racist Ipods a while back. Appartenly apple has "no tolerance" policy when it comes to racist Ipods. Seems kinda racist if you ask me.
J/k either the files arent in the right folders, got renamed, or have a different file extension. Try burning them onto a cd, deleting them from your comp and then putting them back in to itunes from the cd. Should fix the file extension thing.