Some things to check:
- Is Sync to iCloud enabled on all devices for the document / notebook you’re trying to sync?
- Is iCloud still busy syncing things unrelated to MarginNote?
- Have you restarted MarginNote on all devices that should be synced?
- Have you tried selecting the “Manual Sync” button for the document/notebook to be synced (on both devices)?
- Have you opened the “Cloud Sync” menu on all devices? What messages is it displaying? (This is obviously not needed to sync, but perhaps it gives a clue that can help with troubleshooting)
Another option might be to reinstall MarginNote on one or several of the devices. If there are changes made on that device, which have not yet been saved, you should manually back up the documents or notebooks before reinstalling.