Why data is not available in network stream?

[cva cangaran]

server side coding

 TcpClient dupClient = listener.AcceptTcpClient();
 NetworkStream ns = dupClient.GetStream();
 StreamReader sr = new StreamReader(ns);
 StreamWriter sw = new StreamWriter(ns);
 sw.WriteLine("send");
 string da = sr.ReadLine();

client side coding

NetworkStream ns = client.GetStream();
 StreamReader sr = new StreamReader(ns);
 StreamWriter sw = new StreamWriter(ns);
 if(ns.DataAvailable)
 {
  da = sr.ReadLine();
 }
 sw.WriteLine(folder.ToString() + filename.ToString());

Here the server writes the data and waits for reading
but the data is not available on the stream in client side.Why the data sent by server is not available in network stream of client?