OnConnectedToServer not called when testing on same machine

I am trying to make a window appear once a client connects to a server, giving the option to select which team they would like to join.

I am using the following code to set a boolean once the client has connected to the server:

     void OnConnectedToServer(){
         justConnectedToServer = true;
     }

And this code to define the window, and make it appear if the bool is true:

     void JoinTeamWindow(int windowID){
         if(GUILayout.Button ("Join Red Team", GUILayout.Height(buttonHeight))){
             amIOnTheRedTeam = true;
             justConnectedToServer = false;
         }
         if(GUILayout.Button ("Join Blue Team", GUILayout.Height(buttonHeight))){
             amIOnTheBlueTeam = true;
             justConnectedToServer = false;
         }
     }
     void onGUI(){
         if(justConnectedToServer == true){
             joinTeamLeftIndent = Screen.width / 2 - joinTeamWindowWidth / 2;
             joinTeamTopIndent = Screen.height / 2 - joinTeamWindowHeight / 2;
             joinTeamRect = new Rect (joinTeamLeftIndent, joinTeamTopIndent, joinTeamWindowWidth, joinTeamWindowHeight);
             joinTeamRect = GUILayout.Window(0, joinTeamRect, JoinTeamWindow, joinTeamWindowTitle);
         }
     }

But, when I build the project, running a server in one instance and connecting to it in another, the window does not appear. Using the editor as the server, I can see that the client is connected.

I have run in background set to true, so the server is running while the client connects.