I have an 80GB iPod Classic with video, I believe it a 5.5, using iTunes 10.1.0.54. A few months ago I did a complete restore on it, but I've just noticed that it didn't sync all of my audio tracks. I decided to do another complete restore today, but the same tracks do not show up. I have it set to sync the entire library, which is about 47GB worth of music, but it is only syncing about 40GB. It's all checked off, the tracks play fine in iTunes, and the tracks have been on the iPod in the past so I doubt it is a comparability issue (iTunes mentions none for these particular tracks as well).

The tracks don't come up when I use the iPod's built in search function, and when you drop the device tab in iTunes and look at the music section the tracks are not listed there either.

What could be causing this?