Hello Green,
If you're setting NS records to point a sub-domain to an external nameserver host, you'll set up the DNS records in the external nameserver host.
Example:
Let's say you delegate sub-domain1.yourdomain.com to ns1.externalns.com. You must configure zone records in ns1.externalns.com - such as an A, MX, CNAME, TXT, etc... for that specific sub-domain. If you don't configure any records, you won't receive a response - EVEN if you have a sub-domain1.yourdomain.com record within our Zoneedit Zone - you won't get a response because you're delegating it to external nameservers.
Query the SOA for the sub-domain and see which nameservers report back as authoritative.
or
Provide us with the domain/sub-domain to do some queries/checks.