Availability of albums and songs on streaming service is up to Labels and/or Artists.
Spotify wants all the music in the world available to stream and works hard daily to try to convince labels to have their full catalog to stream.
Unfortunately, sometimes, some artists refuse (latest examples are Adele and Rihanna).
It can be also some licensing problems. Licensing rules are not the same from a country to another, that can lead, sometimes, to have albums partially available only for some countries, or songs removed when the license ends.
When it's a licensing problem, usually a solution can be found with a more or less long delay. For artists refusing, Spotify will try again and again to convince them. We can hope that, in a close future, they will accept.
If they want help, users can always try to contact a label and ask why an album is not on Spotify.
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