DNS Subdomain delegation issue

I'd first make sure you're getting ns1/2.provider-dns.com back as an answer to this query:

dig subdomain.example.com ns

You might also want to check that the zone is loaded on the nameservers to which you've delegated. Check that you get a correct serial number back:

dig @ns1.provider-dns.com subdomain.example.com soa

If that isn't working, check with whomever is running ns1/2.provider-dns.com to make sure your zone is being loaded.

In case anyone ever actually reads this article again, and is wondering what the resolution was, I second Jonathan Ross' comment.

On provider one, you added 2 NS records. But you didn't add 2 A records for those 2 NS records.

Without the A records on provider one, the 2 NS records are just names, that have no associated IP. So DNS requests for the subdomain can't make it to the NS for the subdomain. Because there is no IP associated with subdomain's NS record.

It may seem logical (until you understand these record differences, many "stories" you tell yourself make sense) to think "oh the IP will get resolved over at provider 2". The request doesn't make it to provider 2 (unless you of course open up dig or nslookup and connect directly to it).

Hope this helps...