Under the assumption that you’re no longer using iTunes, but instead the new Music app in Catalina, they no longer store the data in .xml format, which is where Sonos currently reads it from. I believe if you go in to Music, and export your playlists as .xml, and store it in the same location as your music, Sonos will pick up the data. I assume Sonos is working on adjusting their application to work with Apple’s change, but I don’t know how long that will be. 

Thank you Bruce I’ll look into it but all my other playlists and my full catalog is available on my Sonos system, it’s just the one playlist that will no longer transfer.

Vince 

There is a limit on the amount of tracks that will be imported as iTunes playlists.  I believe it is 14,000 total. 

https://support.sonos.com/s/article/153

Large iTunes Playlists
If you have more than 40,000 songs in your iTunes playlists

Sonos can index a maximum of 1,000 playlists, with up to 40,000 songs total within all of your iTunes playlists. If you have exceeded this limit there will be missing playlists or songs within playlists when viewing them in Sonos.

Well, 14,000 sounds a little like 40,000.  

Thanks everyone for your comments. If I understand it correctly if the sum total of all the songs added together in all the different playlists is more than 40,000 then some playlists may not transfer and would be missing? My total cache of songs currently is around 10,000 but I have several playlists the one that is missing has 9,300 songs in it, there’s another with 6,500 this one is not missing, the rest probably add up to more than 40,000. Would the solution be to delete some of the playlists from my Sonos to get the lists I want?