About 130 songs won't sync to my ipod?

In my music library there are 1139 songs. In the music under my device, it says I have 1099. There are about 130 songs that don't show up on my ipod. They're in my music library, but they're not on my device and I don't know how to get them to sync to my iPod again. When I go to sync it, I have "Sync Entire Music Library" selected and it says 1139 songs, but after I sync it, my iPod still only has the 1099. They used to be there, but one day I noticed that some songs weren't on my ipod anymore. How do I get them from my library back onto my device? I have a 32g 5th generation iPod Touch.

Please help, some of my favorite songs won't sync. ):