I just had an issue and since there were no clear instructions I thought I'd respond to this thread.
I have had an iphone 5s for years and it's been solid. I thought I'd take the plung and upgrade to 6s. I did the whole itunes backup thing and restore, and then did a 15gig sync of all my spotify playlists etc. I think noticed that in one of my main playlists which has some local files mixed in there was a green arrow "as if" they had been downloaded for offline use, but they would never play. When I selected my MAC as output on the phone they would play. I tried many times to turn that playlist to online, then offline, I tried adding new songs, I tried deleting and re-adding stuff that was there, but basically what I eventually figured out is that local files that had been marked offline post restore to iphone 6s will not play and will not re-sync because my guess is that the ios app thinks that the file already exists. It's possible it could be a "hash" thing ie. when it looks for what it needs to sync it thinks it's there, but it's not post the itunes restore process.
I also tried to delete the playlist that had the local files in and then "restore" it from spofity web app. As soon as the playlist appeared, all the songs already had the green sync icon so I was in the same position. I tried to delete the playlist with the local offline files, but the app never cleaned up the files. I thought I could delete the playlist, then restore the playlist from webapp, but all the files were marked as green immediately as-if they weren't deleted. Maybe a bug causing the issue ?
After many hours of trying and searching, and every attempt to "not" have to delete a playlist and then re-create it, I deleted Spotify on the iphone, waited for it show that the free space was freed up and then added it again, logged in and went through each playlist/file between my old phone and new one to try and get it setup the same way I had it prior. This REALLY messes things up as once the app is deleted and re-added there is no way of knowing what you had previously marked for offline 😞
There is definately an issue when it comes to restoring or upgrading iphones and the way spotify handles the local files. When I did the store all my playlists were perfectly marked for offline as they were on the old iphone..I was soo happy until I figured out the only way to resolve the issue was to delete the app entirely. This is not a viable "upgrade path". Spotify need to identify if something is a local file and marked for offline play and RE-SYNC it regardless of it it thinks the file is there or not...
Spent too much time on this, and there were no posts with any info as above.
I have the latest IOS, latest spotify so it's not a version thing.
I did read somewhere that some people have requested a "clear cache" button, but that spotify would not know how to identify the files as they're given cyptic names when stored/downloaded. Something like this would be most helpful ESPECIALLY if the clear cache process would leave your playlists marked as offline ( for both downloaded and local files ) ie. makes not changes to the status of the playlists, but deletes the storage used so a re-sync can happen.
I've probably downloaded way more than I used to have now as I'm not sure what was marked and what wasn't as I'd been tweaking over time.