Same problem. Seems to happen over and over again with Sonos and Spotify. No problem using Rhapsody or any other music service, which suggests that the problem rests with Spotify. I signed up to Spotify premium only because I wanted to use the service on my Sonos. Will certainly cancel if the problem does not cease.
Had the same. This trick worked: created another playlist (same name). Dragged all files from the old one (the one that never showed up) to the new playlist. It popped up right away on Sonos. Then deleted the old one that dissapeared as well right away. Only thing that still puzzles me are the deleted playlists in Spotify that still show up on Sonos (but are empty)...
Although this is marked as solved, I found it to still be an issue.
Using the Sonos Controller (Mac 10.8; not sure that matters) and browsing the spotify playlists, Select Create New Playlist button. I'll name it asdf. No matter what the name is, it always appears at the very top of my playlists in the Spotify app almost instantaneously. It seems to force some kind of communication to Spotify, allowing playlists to sync.
I don't have a complex spotify playlist structure (a dozen folders, maybe 50 playlists total with less than 100 songs on each). I've heard others in the Sonos forums find sucess by 1) updating music library or 2) re-adding the spotify music service. Not sure where the blame lies, but this has really frustrated me and I too have considered cancelling my premium subscription.
I've been having this problem and did what Valoss suggested. It worked instantly. Then i deleted the new playlist, job done. Not ideal - Spotify could at least have the courtesy to reply to problems like this, their customer service leaves a lot to be desired - but a simple solution.