I can assure that Spotify aims to have all the artists and all the songs on its database. Unfortunately, some artists and tracks are not available at this moment. Sometimes agreements can’t be reached with the artist or label or a change may happen in music ownership.
However, please know thousands of albums and artists are added to Spotify every day. If you can’t find an the band or artist you’re looking for, they may appear soon.
In the meantime, feel free to contact the artist or their label to ask when they plan to be on Spotify. Thanks!
Here's some valuable information on this topic: Content Missing FAQ