Looking at the various releases that I have available to me from a U.S. account it would seem those greyed out tracks are from a Tour Edition of that release. The standard 12 track version release should still be available to you currently.
Sometimes labels and artists will allow bonus, tour editions and deluxe releases to be on the Spotify service for just a short period of time. Other times labels and artists will leave content like this up until the licenses renewal comes up again.
Spotify wants all the world's music, and they're working hard on making this happen for you. However sometimes agreements can’t be reached with the artist or label or a change may happen in music ownership.
Spotify adds thousands of albums and artists are added every day. If you can’t find an artist you’re looking for, they may appear (or reappear) soon. In the meantime, feel free to contact the artist or their label to ask when they plan to be on Spotify.