Offline Listening not possible despite premium

[sasalida]

Hey, 

I registered a massive consumption of my data volume caused by spotify. I have a premium account and downloaded my whole library, so every song has a green sign. As soon as I click in settings to only listen to downloaded songs when offline, no song of my library is played anymore. 

How is this possible? My data volume is reduced every month bc of spotify even tho I'm not searching for new music when not in wlan. I just don't get it, maybe someone can help me.

Thanks in advance.