Ok, I found the problem. iTunes has different catalogs for each country. The script searches the US catalog. When you use the iTunes software, you will always see the catalog of your own country.
The single you are mentioning is not in the US catalog, but it is in the Great Britain catalog.
http://itunes.apple.com/us/artist/lawson/id2627885
http://itunes.apple.com/gb/artist/lawson/id2627885
I guess we would need different country versions of the script to cover that, similar to the Amazon scripts for Mp3tag.
As for the Discogs script, please report that in the respective forum thread. But I never had problems to find things there. I use that scripts very often. However, it is very sensitive for little spelling mistakes.