Today I set up a new sub-server that would not work. After checking around I found that IP address in the DNS record was the systems internal IP rather than the external.
In the Module Config | Network Settings | Default IP address for DNS records is set with the external address. I checked the configuration for the domain in question and it too has the external IP correctly entered.
What I did find was the parent server had the internal address entered as the external IP address on the edit virtual server page (an artifact from a server move).
It seems to indicate that the sub-server receives the IP address from the parent REGARDLESS of the IP address used, whether from the module configuration or a manually entered IP when the sub-server is created.
Chris