If Gneo has become out of sync on your iPhone and iPad, you can select 'Re-set' in Settings to do a full re-sync.
Note: Please ensure that you re-set the device that does not have the missing tasks/notebooks as your tasks may not have synced to Cloud Gneo and could be deleted if you re-set the device with the new tasks. If you are concerned, please take screenshots of your tasks prior to re-setting.
Go to Settings (tap once on the middle button and Settings is on the top right), and scroll down to 'Reset Gneo'. Tap this and select 'Yes'.
Then, go to the List view and pull down the screen to refresh. All of your tasks and notebooks will now be re-synced to Gneo.