A restore that gets rejected almost always comes down to one of a handful of causes. Here's how to work through them.
When JW Library refuses to restore a backup, it's rarely random. The app is strict about the exact structure of a .jwlibrary file — it's a ZIP archive containing a specific database and manifest, and if anything about that structure is off, JW Library rejects it rather than risk your study data. Here's what typically causes it, roughly in order of how common each one is.
If the backup was interrupted mid-transfer — a dropped connection while emailing it to yourself, an interrupted cloud sync, a cable disconnect — the file can be incomplete without your file manager showing any obvious sign of it. Re-download or re-transfer the file and try again before troubleshooting anything else.
Some messaging apps and cloud services rename attachments or strip extensions. Check that the file actually ends in .jwlibrary and not .zip, .bin, or nothing at all. Renaming it back usually resolves this instantly.
If the file came from a third-party tool that builds its own .jwlibrary output, small deviations from JW Library's exact expected format — how the archive is compressed, whether a required internal file is present, how the integrity hash is calculated — can cause a silent rejection with no useful error message. This is a common failure mode for merge tools that reconstruct the file from scratch instead of building on a real JW Library database.
Less common, but possible if the original export was interrupted or the storage it was saved to had an error. If you have an older backup from the same device, try restoring that one to confirm whether the problem is the file or something else.
This specific error is especially common when merging backups from two phones (or a phone and a computer) into one file, since that requires rebuilding parts of the file rather than just copying it. JW Fusion is built specifically to avoid the format issues described above — it merges directly on top of a real JW Library database rather than reconstructing one, so the output restores the same way an unmodified backup would.