If sync stopped working, try the following troubleshooting steps, in order.
Make sure you are indeed connected to the internet by visiting a website in Safari.
Also, make sure you are logged into iCloud and iCloud Drive is turned on, in System Preferences for Mac and Settings app for iOS.
Wait a While
Most sync failures are temporary. If you just wait a few minutes to an hour, sync will resume by itself.
You can monitor the status of GreenBooks’ sync by going to the iCloud tab inside GreenBooks’ Preferences (Mac) or Settings (iPhone).
Turn Sync Off and On
If sync does not resume after after waiting a while, you can try turning iCloud sync off and on again on each of your devices. You do that by going to the iCloud tab inside GreenBooks’ Preferences (Mac) or Settings (iPhone).
Reset iCloud Sync
In the rare case where turning sync off and on doesn’t work, you may have to reset the state of GreenBooks’ iCloud data on Apple’s server.
- On each of your device, turn off iCloud sync inside GreenBooks’ Preferences (Mac) or Settings (iPhone).
- On one of your devices (the one with the latest data), go to the Advanced tab in GreenBooks’ Preferences (Mac) or Setting (iPhone) and click “Reset iCloud Data”.
- On the same device, turn on iCloud sync and wait for it to complete successfully. Enable sync on the remaining devices.
If you are still having trouble with sync after trying these steps, please contact support.