On both my iPad and iPod, you can save (+) or remove (-) a song from "Your Music" while using a playlist. But now it doesn't work. (I reinstalled both iOS Spotify apps) It still registers the change but if you look up the song under artist you will see nothing has changed. I created a test playlist of just two songs. Now it has only the first song "saved" on the iPad, the second song on the iPod, and both selected on the PC. Checking out the songs under Artists suggests that only the PC playlist was correct. Perhaps the last iOS update has a bug.