A lot of the time it's with th artist/label to arrange for tracks to become available on Spotify. It may be worth sending them an email and asking if they are considering releasing their titles on Spotify?
For more details on content and why some content may be missing see here.
Please use the search feature before posting a problem/bug/idea. What's a Spotify Rock Star, and how do I become one? Using Spotify Since March 10th 2009 Twitter: @mechanimal82