Happens to me more often than I'd like.
Aside from making sure all the disc and total track numbers are the same, you can try to force iTunes to recognize the songs as all part of an album by selecting them all, "Get Info", intentionally misspelling the album en masse, then correcting the album name en masse. Sometimes you have to do this even when it appears the album name is the same but iTunes thinks otherwise.
Oh, and make sure they all have the same album artist.