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Sometimes, the following factors contribute to the loss of Offline Music;
Since the music is stored within the cached portion of the iOS filesystem, sometimes files necessary for offline playback can be lost unintentionally. Due to the Premium nature of Spotify's music catalogue, offline music will not play if its been tampered or extracted - this pointer won't apply to a standard and secure iOS device.
Please attempt to re-sync your music. Should the problem re-occur often, please refer back here and we'll perform further troubleshooting.