Smooth synchronization requires Internet access on all relevant devices so that data can be exchanged.
Check that you are logged in to all devices with the same cabuu account.
Note that you shouldn't use the app on different devices at the same time with one account, i.e. add or edit words in a list that are currently being learned on another device.
The sync is automatic when you start the app, or you can do it yourself: At the bottom of the main menu, tap Account → fix problems → Sync device.
