Because sync runs through your iCloud rather than through us, almost everything that goes wrong is on the iCloud side, and almost all of it is one of three things.
1. Same Apple ID on both?
This is the cause more often than the other two together, and it is easy to miss on a device somebody else set up, or an older iPad signed into an account you stopped using. Two Apple IDs mean two separate vaults that will never meet.
2. Is sync on in the app on both devices?
Sync is off until you turn it on, and it is a setting on each device rather than something that travels with the vault. A new device is a device with sync off.
3. Has it had a connection?
The app works offline, which means changes made on a plane sit on the device until it reconnects. Open the app with a connection and give it a moment. On a large vault the first pass after a long gap is not instant.