Playlists randomly stop during playback in Airplane Mode / Offilie Mode

[OmRunYoga]

I teach yoga and pilates, and put my IPhone in Airplane mode during classes so phone or text sounds don't interrupt. I make class playlists availalble Offline, and switch Spotify to Offline mode. Every class, but at no certain time in playback, the playlist just stops. Then I need to click the "Shuffle" button again to get it to start, and then it's often on a song that has already played. What do I need to do to have consistent, continuous playback using my playlists? Thank you.