Hi,
I had the same problem of “Documents and Data” syncing between Mac and iCloud. Turning off and on again the checkbox of ‘Documents and Data’ in System Preferences didn’t work as a solution and Console.app showed lots of errors in “ubiquity-digest.log” such as “there was a http error 503 while talking to iCloud reason: Blocking request by ‘my device name’ in progress; try again later.”
This is my solution:
1) Make sure you have backups of all your documents and data
2) Turn off the checkbox of ‘Documents and Data’ in System Preferences > iCloud
3) Delete ~/Library/Mobile Documents Folder
4) Delete ~/Library/Cashes/com.apple.ubd Folder
5) Delete ~/Library/Preferences/com.apple.ubd.plist
6) Delete all keys and certificates named ‘com.apple.ubiquity.*****’ in the login keychain of Keychain Access.app
7) Turn on the checkbox of ‘Document and Data’ again in the System Preferences > iCloud
8) Wait several minutes or open an application that uses documents in the iCloud to start syncing between Mac and iCloud.
You can see the detailed process in the “ubiquity-digest.log” of Console.app.