I recently updated to the Spotify Premium and I cant download music for I hear when I'm offline (on my iPhone), and when I go on my account settings (www.spotify.com) and I search my offline devices it doesn't appear anything .
Try logging out and back in to the Spotify app to update the changes on the system. Sometimes doing a clean reinstall takes care of it. If you still can't download a thing, contact the Spotify team here so they can take a look at your account.
If you get an automated message from the system, simply reply back to that email.