I experienced this several times now for months or even years, an iCloud not doing its job. This primarily is syncing your data over your devices. I use this heavily with the notes app or with anything related to HandOff (copy & paste within different devices).
I was able to identify the processes that do not work properly, so now I am able to just kill them. No worries, they will restart immediately and then everything works fine again.
This is of course only for desktop machines running macOS - I never had issues with iOS syncing between other iOS or iPadOS devices.
Create a new text file wherever you want. I recommend to use the Documents folder as this might also be synced over your devices and then you have the script automatically available on all of your Macs. Name the file fixicloud.sh. Open the file with a text editor of your choice and add the following:
sudo killall mds_stores && sudo killall cloudd && sudo killall bird
Save the file. You might have to enable execution rights for it, so open a terminal window and navigate to the folder where you put the file. Execute
chmod +x fixicloud.sh
Finally to fix the issues run the script with
You have to provide your Mac's account password to run it. You now should see the iCloud data syncing again on your Macs.