Missing or unavailable music is almost always a decision by the artist/label to withhold the content from Spotify. Sometimes it's temporarily unavailable while they sort out legal issues or correct metadata, in which case it'll be back up as soon as they resubmit it to Spotify. If you're looking for some more information, it might be a good idea to contact the artist or label directly and ask why the music isn't on Spotify. You might also be interested in supporting this idea, which suggests a simpler method for requesting albums missing from Spotify. For more information on missing content, please check out this support article.