If you use the DNS Administration Tool to add NS records to the zone file, existing NS records may be dropped when you restart the DNS service.
When the DNS Administration Tool casuses the server to add NS records to the zone file, they are added at the beginning of the list of NS records for the subzone and are supposed to have the same authority level as other records in the zone. However, the DNS server assigns a slightly lower authority level to the old NS records on startup, causing them to be dropped.
To workaround this problem, DON'T use the DNS Administration Tool to add NS records. Edit the zone file manually to insert the new NS records.
NOTE: You must stop and restart the DNS service for the changes to become available.