1. Check the account
Confirm that both devices are signed in to the same Send Listy account. Account-backed lists are synchronized through the authenticated account flow.
2. Check whether the list is local-only
A truly local unsynced list has no sync identity and is intentionally treated differently from a list that has already been backed by the account.
3. If the problem follows a deletion
Send Listy treats deletion as authoritative for account-synced account lists. A stale device should not recreate a list that was deleted on another device.
Related guide
See Sync Lists Across Devices and Delete a List.