This just started happening to me (or maybe it was happening all along and I didn't notice), but I was trying to backup a Final Cut Pro library to my iCloud Drive, and waited days with no data actually being transmitted. Watched the activity monitor. Turned iCloud Drive, desktop/documents on and off. Tried to move a file from my other MacBook to the same iCloud Drive and now nothing will sync. The only thing I touched was the "optimize Mac storage" I turned it off, because I noticed it was downloading stuff to my local machine that I didn't want there, I wanted it kept up in the cloud storage, so I figured I would just manage it manage by using right-click "remove download". That is when I noticed this happened. So I put it back to default, turned iCloud off and on again, and still it's stuck. Paying for 2TB of data that I can't use!