Some songs will not transfer to my Ipod

This is wierd. When I sync my ipod to my playlists, some songs will not sync. The files are there and are located in itunes. Its strange because, sometimes when I sync them they will sync and sometimes they will not. It is usually indcated in the music list with a circle beside it. On the ipod it just says "not playing" and skips to the next song. I have a few that have the circle that do play on the ipod. All of these songs come from a CD where other songs were also taken that do play and some were even purchased direcly from itunes. Any ideas? By the way, all of them will play on the laptop.