Songs on iOS 8.1 say they are downloaded to the device, and available offline, but will not play. This is onl for the songs that I added to spotify on my computer that are not in the Spotify library. Previously, when I would sync these to my phone from my computer, they would still play (when i made them available offline on the phone). Doing that now, they show up on the phone, but won't play.
Edit: Reinstalling/ Updating the spotify app worked.