some messages not syncing to macbook

See the following Apple articles:

Use Messages on your Mac

Use Messages on your Mac - Apple Support

Use Messages in iCloud

Use Messages in iCloud – Apple Support (UK)

Sync content between your Mac and iPhone or iPad

Sync content between your Mac and iPhone or iPad - Apple Support (NZ)

Access your messages on all your Apple devices

Access your messages on all your Apple devices - Apple Support (MY)

I read the part of the Messages user guide called “Access your messages on all your Apple devices”.  It says:

The problem I was having is that this is what they think it looks like:

But this is what I was seeing on my MacBook Air 2020:

The “Enable Messages in iCloud” selection and the “Sync Now” button were just not there.  I fiddled around and found this:

1. Go to System Preferences > Apple ID > iCloud > iCloud Drive
2. Click on Options…
3. Make sure the following are selected:  

imagent.app

IMTransferAgent.app

sociallayerd.app

Out of an abundance of caution, since I did not recognize these and they looked kinda suspicious (they had weird names and did not have icons) I had deselected these after reading a few threads in Apple Support where others were wondering what they were.  Just for laughs, I turned them back on just now and the “Enable Messages in iCloud” selection and the "Sync Now" button magically appeared.

I was having the same problem on my new M1 iMac, but it fixed itself mysteriously.