I have a recurrent problem with Spotify on iPad. I have a premium account. I have downloaded several albums for offline playing. All the songs in the album then show the green icon that indicates that they have been downloaded. However, about half won't play when offline -- they don't say "not available offline", rather they appea to play but no sound somes out. And when a playlist hits one of these songs, it stops -- it doesn't go on to the next song.
I've tried deleting all offline content but the problem keeps on recurring.