Sometimes a client complains that a Backup/Restore missed a collection.
In this situation, we often find that the collection was not fully replicated to all of the servers in the original cluster. The replicas must be distributed across all of the servers for backup to capture them reliably.
The remedy is to fill in the missing replicas. See Some of our replicas are missing! for a comprehensive example.
Do not hesitate to contact the SearchStax Support Desk.