I’m going to guess that you’ve done remote control before. You’re probably facing a name resolution issue. I’m picturing two machines getting their network configuration information from a broadband router. They’re able to reach systems on the Internet, on the other side of the router, just fine. This side of the router, though, there’s nothing managing names for you.
You can work around that by using the IP address of the second device instead of its machine name. Or create a hosts file entry.