I also can see Sonos from spotify app then try connecting. After about 10secs of trying to connect it reverts back to phone. Never had this problem before. Cannot use Sonos app as this has insufficient functionality to reach all my music in Spotify.
Also I have always had problems changing volume levels through Spotify app when connectected to Sonos. Get round this by openning Sonos app to change the volume.
I only got premium for the Sonos connectivity so may have to leave. Google music with Sonos works effortlessly. I am trialling them both.
Re: Can't connect to Sonos players in Spotify Connect
Not sure if this helps at all but I just recently had this problem yesterday and fixed it just maybe two minutes ago I wasn't able to play music on my Sonos from the Spotify app on my phone but I could on my tablet and while trying to troubleshoot I opened up the Sonos app on my phone which I knew would play already and went into my settings to check for online updates and it was one of vailable after the update finished I closed it out went back into the Spotify app and was able to select the Sonos and now my music is playing. Hope this helps