There should be a forum/poll where us users can let know artists/labels and Spotify itself the music we would like to have. Like tagging the artist profile to the subject or something like that. There's a lot of artists with almost all of their album but one missing, often the most popular
Please add your kudos and comments there, if you haven't already. 😉
Spotify is always trying to add as much music content as possible. If a track/album/artist isn't available (in a certain country) it's mostly the decision of the artist or their label. For more information about that topic, please have a read at point 5 of the official FAQs respecting additional music requests here.