Why do only some of my playlists get synced to the cloud?

Hello,

I've recently subscribed to Apple Music, hoping to be able to access all my Apple Music playlists (currently on my iMac) on other devices like my iPhone. However, of the 40 or so playlists on my iMac, only 20 copy across to my iPhone. I've logged in to my online Apple Music account via a browser and discovered that it's only those 20 playlists that have copied to the cloud; my other playlists are not being synced.

As far as I can tell, my music is all there in the cloud (and on my phone) but I use Playlists for listening to music and most of my key ones aren't there.

I tried deleting a playlist on my iMac and then did an "Update Cloud Library" and the change was replicated in the cloud withon moments, but the deleted playlist wasn't replaced by one of the missing ones, so it's not a 20-playlist limit I'm encountering; it seems that half my playlists are simply not being recognised to upload to the cloud.

Can anyone help?

Many thanks,

Jon