I know it sounds simple, but it's often overlooked. Could you try restarting your device? If that doesn't help, try a quick clean reinstall of the app. Just follow these steps.
I'd also recommend updating to iOS 12.2 :)
If you're still having trouble, could you let me know the exact version of Spotify and the exact version of the operating system you're running?
I'll see what I can suggest next!