Offline mode stops after a few songs

[c3130391]

When I go into offline mode, I can play about 6 or 8 songs before it says that the songs are unavailable so the only way I can listen to my offline songs again is to sign out of spotify, sign back in and select offline mode. Then repeat once it stops playing my songs.
I have plenty of space left on my phone. I've tried turning it off and on again. I've tried re-downloading the playlists. And it's not specific songs because of I go to replay a song I've just been listening to, it says unavailable.
So annoyed that I pay for a service that I'm not getting. Help!