The duplicate search is not very sophisticated. It won't pick up that "Rotate My Tires" by Articulated Weasels is the same song as "Rotate My Tires" by The Articulated Weasels or "Rotate My Tires" by Articulated Weasels with Don Ho.
ID3 tags, especially if they were ripped by another program and later imported into iTunes, can also contain unprintable characters that will mess up the matching even if the information appears to be the same.
Still, it seems odd that you would have no duplicates if you know there are many.
As an alternative to the duplicates view, you could try sorting all the songs by clicking on artist, then name. It's more work but better than nothing.