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.
Any content that has disappeared or been removed (the tracks will still be in your playlist, just hidden because they are unplayable) is normally down to the record label or rights holder. Sometimes content is removed due to licensing changes, transfer of ownership or due to a request from the content owner.
Have a read at the Music FAQ topic for a little more information on how music is added to Spotify and a request form. Skylar