First, ensure that you are signed in with the same Apple ID on all devices and that iCloud and iCloud Drive are enabled for Book Tracker. If iCloud is enabled but synchronization is still not working, proceed as follows on all your devices (one device at a time):
- Open the app settings and execute a backup in the “Backup & restore” section.
- Open the device Settings and in the iCloud section disable iCloud for Book Tracker.
- Open Book Tracker and check that no books are in the library.
- Re-enable iCloud for Book Tracker from the device Settings.
- Open the Book Tracker and check that the library populates again.
Perform this operation on all your devices. From this point on, synchronization should resume. If you notice that some books are missing, you can restore a backup in order to have all your library again. If the problem continues, you can contact support for further assistance.