Episodes of TV Shows sort alphabetically instead of by episode number. Used to sort in episode number but no more..

My iPod (5th Generation) used to sort all of my tv shows by tv show, season and episode number. Now it just sorts by tv show, season and then alphabetical order for episodes. This is very annoying with 800+ episodes and not knowing what order your watching them in! It used to always sort them in the right order, but then an update to itunes before christmas last year changed all that. i never got around to posting a question about this because I've just been busy since it happenned. But I was hoping does anybody know a solution to resolve this problem and get them sort by number. Oh and I've already tried tagging them with track number, and it doesn't work.
Thanks,
Joe