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, such as AC/DC and Led Zepplin who have chosen not to be part of Spotify.
Any content that has disappeared or been removed (the tracks will still be in your playlist, just hidden because they 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.
Spotify upload all music which is made available to them for our enjoyment! I would suggest you have a read at the missing content FAQ as it will probably answer most of your questions!