Aqua Timez is one of Japan's best bands and its completely missing from the catalog which is a bit depressing. If you don't believe me, check out Aqua Timez Gravity 0!
Spotify tries really hard to get music available on their service, but in the end the availability of music on Spotify is up to the artist and their music label.This thread by Spotify staff explains why some music is not available:https://community.spotify.com/t5/Content-Questions/I-want-an-artist-song-on-Spotify/td-p/1136005
Aqua Timez, AKB48, Supercell, Scandal.
Used to be available, but not anymore.