downloaded song no longer available offline suddenly

[asandmusic]

I download a complete playlist, I go into offline mode, a few days later some downloaded songs appear as not downloaded anymore. They played just fine up until that point. Did not make any changes to playlist of online status.

How do some songs become "un-downloaded" all of a sudden? 

I am usually able to go back online and download them again but I don't understand why they are undownloading in the first place. Again, this is just a song here and there in a playlist, not all songs in a playlist.

thanks!