With this issue, it seems as though it might be a content licensing issue between Spotify and a content distributor or label.
Spotify wants all the world’s music on Spotify. However, some artists and tracks aren't currently available on Spotify. Sometimes agreements can’t be reached with the distributors or labels, or a change may happen in music ownership causing the music to be removed or not shown.
Remember though, that thousands of albums and artists are added to Spotify every single day. If you can’t find an artist you’re looking for today, they may appear (or reappear) very soon.
This article from Spotify Support explains licensing more in depth, and how it relates to Spotify.
In the meantime, feel free to contact the distributor, the artist or their label to ask when they plan to be on Spotify. Moreover, there's still a lot of music on Spotify, so you might end up finding some music that you enjoy with the existing collection.
Sorry for any inconvenience, and thanks for posting to the Community! Tim