CC-SG uses DNS when a device is added using a hostname.
Make sure the DNS server is listening on the same address that CC-SG has configured for the DNS. The DNS is configured in Administration > Configuration, Network Setup tab. See Configuring the CC-SG Network.
Refer to http://technet.microsoft.com/en-us/library/cc783049(ws.10).aspx for details. These instructions are an example for a Windows DNS server, where CC-SG is configured with an IPv6 address for DNS.