On my iPad via the Spotify Settings - Devices option I can see my various Sonos zones/devices. But when I select one of the Sonos devices Spotify will not respond and change its output to that device. I will happily change to another computer or even to the iPad itself, but not to the Sonos device.
This behaviour is also happening on my Mac Air.
It all worked originally but seems to have stopped now. My Sonos software is up-to-date. Any advice would be appreciated.