Sync error in playlists

[xavidoo]

Hi

I've got my playlists created but when I make them available offline on my iphone 5s there seems to be a problem:

At first, when offline and synced all songs of the playlist are available and I can listen to them perfectly offline. However, after switching off and on the spotify app, being still in offline mode, some of the songs of that same playlist are unavailable as if never synced offline.

How is it possible that I can listen to them in offline mode but after closing the app they become unavailable?

Any help, please?

Thanks a lot