Following this as well. I looked back to the spotify playlist I set up on my PC. There were 3 songs that were linked to my iTunes library instead of being pulled from streaming spotify. I deleted those off and it showed up INSTANTLY on my sonos / spotify playlist on my iOS phone. Good luck, to the right of the song on the sonos desktop client there is a little music note notating iTunes or local library, those are your trouble tracks!
What worked for me (from my iPhone) was simply removing the Spotify music service, leaving it all alone for a few minutes, then re-adding it. It failed a couple of times to re-add the service, but once it did all of my dozens of playlists showed up immediately.