A couple times in the past week when I've powered down my Samsung Galaxy S5 and then powered up again, I found that my Spotify playlists that were previously Available Offline, were no longer Available Offline. Checking my SD card it still shows that the files for the songs are on there, but I don't know how to re-connect Spotify to those files. The only way to get them back is to completely clear out my data and then re-load all of my lists which takes many hours as it is about 60 GB of music.
How do I stop this from happening? If it does happen, how do I get Spotify to recognize the files that are already downloaded to my phone so I don't have to re-load them again?