Try un-linking the Spotify account in the Sonos app and re-link. Is there just the one Spotify account in your Sonos profile?
Right, I finally figured this out.
If you remove Spotify from the sonos app but still have it linked in Alexa. Alexa will still play Spotify if you you ask her to. You have to make any updates in Alexa.
What my problem was is that I have 2 Spotify accounts linked to my email address. One that's linked with Facebook and a stand alone one.
When I linked the account with Alexa I used my email address and it defaulted to my Facebook one, which only had the free version.
I had to relink Spotify in Alexa and use my Spotify username to force it to the account with the premium subscription.