Are you logging in with the same details? Sometimes it's easy to be logged in as Premium using a Spotify ID, but use a Facebook ID to login on another device. This will treat that Facebook login as a new user.
You can also see if you've reached your device limit (or also to see if devices are correctly showing on your account) by going to the following webpage and logging in: https://www.spotify.com/account/offline-devices/
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