I was having this issue with one database, and am not on Catalina (am using Dropbox and NOT iCloud). I contacted support and they suggested I “clean” the Database from the Mac in Preferences > Sync. I think what it does is reuploads a new database into the sync store. Before this I disabled syncing of that database on all other devices. After cleaning it I turned sync back on and let each one sync fully one at a time. It fixed my syncing issue. I had been using that sync store for a very long time, so somehow it got corrupted. This works well in my experience you just have to have good bandwidth and let it take it’s time. You can probably find more official help on this in the forums or their manual.