Flex :: 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.

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);

[code]....