What hinders NetStream.onPeerConnect from being triggered?

I'm using Adobe Stratus (now renamed to Cirrus) to build a p2p application. Inside the application, I used NetStream.onPeerConnect callback function and expected it to be triggered every time when a peer is connected. However, it always failed with my friend A while strangely friend B managed to have the function called without any problem.

I was wondering what could be the cause to this issue?

Here are how the code pieces look like.

First of all, create a NetConnection.

netConnection = new NetConnection();
netConnection.addEventListener(NetStatusEvent.NET_STATUS, netConnectionHandler);
netConnection.connect(SERVER_ADDRESS+DEVELOPER_KEY);

Secondly, create NetStream upon NetConnection successfully connected.

private function netConnectionHandler(event:NetStatusEvent):void{

    switch (event.info.code){
        case "NetConnection.Connect.Success":

            sendStream = new NetStream(netConnection, NetStream.DIRECT_CONNECTIONS);
            sendStream.addEventListener(NetStatusEvent.NET_STATUS, netStreamHandler);

            var sendObj:Object = new Object();
            sendObj.onPeerConnect = function(subscriber:NetStream) : Boolean {

                trace("[onPeerConnect] far id: " + subscriber.farID);
                return true;
            } 
            sendStream.client = sendObj;
            sendStream.publish("file");
            ......

Thirdly, here's how I build the connection between two peers

    receivedStream = new NetStream(netConnection, farId);
    receive开发者_Go百科dStream.client = this;
    receivedStream.addEventListener(NetStatusEvent.NET_STATUS, incomingStreamHandler);
    receivedStream.play("file");

Please help enlighten me. Thanks!

It turns out my friend A is behind a symmetric NAT. I'm thinking to setup a TURN server for us to build a successful connection.