Any content that is missing is normally down to the record label or rights holders with holding those tracks from Spotify, sometimes to push up physical and download sales. There are also some big names, who have chosen not to be part of Spotify currently.
Have a read at the Music FAQ topic for a little more information on how music is added to Spotify and a request form.
just to say that Come - gently down the stream - is on Spotify. I don't know if it's a problem of my browser but i found releases upped on Spotify searching the name of the songs instead of artist or album.