Why doesn't my iPod Classic show all the songs it holds?

I recently bought a new iPod Classic 120GB, mainly to use on my Numark iDJ2 but also for general use.

I have it set so that all my iTunes songs are automatically pulled over to it and when I look at its contents on iTunes all my songs are there. However, if I unplug it and then search through it manually, some artists don't appear. (I haven't yet found a pattern.)

The only reason I mention my iDJ2 is that I have Numark's 'Librarian' software installed on my iPod so that the iDJ2 can categorize and play all of my songs. I don't see how it should be responsible for this problem though.

Any help much appreciated and let me know if you need any more info.

Could it be that some albums are tagged as "Compilation"? In this case the artists in these albums are not shown in the "Artists" submenu... Uncheck the "Compilation" check-box (or something like that - I don't use iTunes!) in the Information tab and they magically appear in the list.

PS. When you select an artist that "has" a proper album, the songs in the Compilation albums are also shown.