The Shuffle option isn't available in a Radio Station or your Daily Mix or your Discover Weekly or the Release Radar playlists which require you to give feedback on them so Spotify can offer you with a more accuracy the music you like to listen to :)
Hope it help!
LoxerRock Star 16
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Shuffle being forced in the mobile version is a huge drag and generally stops me using my phone for Spotify because of the lack of control. I'm seriously underimpressed with this aspect of the service.