Music downloaded but still after a while goes dark

[Tomstud12]

Plan

Premium 

Country

 Ireland

Operating System

Android 8.1

My Question or Issue

I have my music downloaded and have premium when I am playing music on my phone it works perfectly no problem.

but when I use it on my other device android 8.1 the music is downloaded it works and than after a while like an hour or 2 while on Spotify. The music will turn dark even thought it’s Downloaded and it says I need to connect to internet to check if I have premium and I have to do this every day every hour or two when I listen to music why is this?