The first thing is to make sure you're using at least version 1.1.1 of
Responder (actually, 2.0.x is the most recent version).
The next thing to check is the Get Info menu item in the Finder to see if
the System file is locked. Responder writes the name of the client
Macintosh to a resource in the System file. If the System file is locked,
Responder is unable to make a permanent change to the System file.