Instantiate fails with no errors

I am trying to instantiate a GameObject into a scene after I receive a response from my server.

Currently I send the request to the server, receive a response and then call the instantiating method.

Everyt$$anonymous$$ng works right up until the Instantiate line, where I get no errors and not$$anonymous$$ng happens. Even the Debug line above works, so I know it is reac$$anonymous$$ng the method.

Any ideas? Is it to do with the Async Callback I have when I receive the response from the server.

NetworkHandler.cs

 public void ReceiveData(IAsyncResult result)
     {
         int receivedDataLength = socket.EndReceive(result);
         string receivedString = Encoding.ASCII.GetString(receiveData, 0, receivedDataLength);
 
         CheckInput(receivedString);        
     }
 
     void CheckInput(string input)
     {
         Debug.Log("Received: " + input);
         switch (input)
         {           
             case "Fleet Created":                
                 FleetHandler.instance.FleetCreated();
                 break;
             default:
                 Debug.Log("Input not recognised");
                 break;
         }
         socket.BeginReceive(receiveData, 0, receiveData.Length, SocketFlags.None, new AsyncCallback(ReceiveData), null);
     }

FleetHandler.cs

 public void FleetCreated()
 {        
     Debug.Log("Fleet created");
     GameObject fleet = Instantiate(fleetItem, fleetDisplayList) as GameObject;
 }