The Symantec Endpoint Protection Manager seems to have a few quirks.
While trying to push out a machine using the Find Unmanaged Computers tool, I kept seeing the machine in the unknown computers (example shown below). Pushing clients out to these machines would consistently fail. After disabling the firewall, I hit Search Now and tried the scan again. Once again, the machines would appear in the unknown computers tab.
On a whim, I closed the Find Unmanaged Computers tool and reopened it, filled in all the information, and hit search now. Much to my surprise, it appeared under the correct tab and I was successfully able to push out and install the clients.