You might try doing a complete reinstall. It worked for me when I was having problems:
1. Delete the Spotify application. To do this, press and hold the application until it hovers, then press the X symbol 2. Restart your iPhone to refresh 3. From the Home screen, choose Settings > Safari. Tap Clear Cache. 4. Install Spotify again from the AppStore 5. Please be sure that you have iTunes closed on your desktop 6. Open Spotify on your computer and log in 7. Connect your phone to the same Wifi network as your computer 8. From the Home screen of your iPhone select Settings > General > Auto-Lock > Never 9. Open Spotify on your iPhone and log in 10. Your phone should now appear under “Devices” in your Spotify desktop, select the phone. 11. Choose the playlists you would like to sync for offline use on your phone.