- Check the amount of offline devices on your account settings site again (3 devices with downloaded music are allowed) and make sure the shown devices are your ones
-Make sure you have granted the Spotify App access to your phone storage
-Make sure you have got enough free phone storage to download the music you try to
-Make sure the songs you have choosen to download are fully downloaded before you stop the Spotify App from running.
If these tips couldn't help you I would recommend you to write the Spotify Support Team.