Windows DNS not picking up change to external website A record

I have pointed an external website to a new IP address.

When I query the domain from an non-networked computer or device it picks up the change.

I've run dnscmd /clearcache on both DCs several times now and they keep getting the old IP.

There is nothing overriding the external IP on the DNS servers (no zones with the same domain name as the one I'm trying to update.

If I query the same external dns server that my DCs use for forwarding, for the url on a non-networked device I get the new IP. So the external DNS server has picked up the change.. but my DCs seem to be stuck with the old ip even after clearing the cache.

Edit: On further investigation (nslookup thedomain. externaldnsserver) the external DNS server is still serving the old ip address to my DC... but it serves the new IP address to other devices. I guess the problem (if it is a problem and not 'design') is on their DNS server.

Edit 2: The external DNS server in question is now returning the new ip. I will leave this question undeleted in case there is any value in it being left... unless anyone disagrees?