Here is a document that you may, or may not, have seen that covers this issue: Agent Deployment Results in error: "Unable to contact the specified machine" 1087
I would also check to see if you can use NSLOOKUP to resolve the host name and the IP address back to the same machine.
The log file might also be helpful. Whenever you push an agent from the core server, it is logged in "C:\ProgramData\LANDesk\Log\raxfer.log" on the core server.
Please let us know if that helps.