Missing or unavailable music is almost always a decision by the artist/label to withdraw 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.
While it might be a good idea to contact the label directly and ask why the music isn't on Spotify, you can also fill out this form to let Spotify staff know of your interest.
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