Why does my ipod keep putting my music in an Unknown album folder when i try to keep artists etc together ?

Hello everyone.

I'm after some help with a problem i'm having with my ipod touch 64gb.

For some reason when i try and group an album together or put a random song into an album, the songs keep disappearing into an unknown album folder.

They are still on the ipod but the only way i can find them is if i search through the song titles.

Don't understand why it keeps doing this as it's never happened before and i make sure everything if grouped correctly (i.e album artist etc).