There could be a lot of things going on. If you double-click on one of your registered servers that are listed by name does it connect successfully? If so I would take a look at the properties of the registered server. You could have something like SQL listening on a specific IP that is not registered to a host name in DNS so when the server is added to registered servers it just lists the server name in the label, but it is still using the IP address to do the actual connection.