Apple Music does NOT sync between devices

I think I have found a solution for this, at least in my case. Out of the 20+ playlists I have in iTunes about 8 would not sync/upload to Apple Music on any of my devices. Apple Support kept escalating the issue but could not figure out why and had "never seen this" before. Randomly playing around with the playlists out of frustration I found in each playlist one single song that had the double cloud symbol with a strikethrough stating "duplicate match already found in the cloud not synced" (or close to that verbiage. I searched my library and found I already had that song and was not sure where this duplicate came from but as soon as I deleted the one not synced, the playlist instantly appeared on every device. This worked in every playlist I was missing. I have no idea how a song was duplicated and not synced and Apple did not know why that would block a whole playlist from syncing to Apple Music but it worked for me. Hopefully that helps.

If you are sure you have Sync Library turned on, then see this article:

If songs are missing from your music library after you turn on Sync Library - Apple Support

I'm having nearly the same issue. I subscribe to Apple music but only about 1/4 of the playlists I have created in iTunes are showing up on my iPhone, HomePod and AppleTV. I have forced "sync library" in iTunes multiple times, signed out of my account and back in on all devices, restarted them and still I am missing over half of my playlists through Apple Music.

There were some issues with Apple Music in recent days. Apple System Status says it's ok now, but I have same problems ever since.