C# Network sending problem

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C#

network

Hi guys! This is a pretty tough question, I think. Sorry that its pretty long, too.

I have a problem with communicating with a lab apparatus. For the interface to work, I have to send back acknowledgements. That seems to be the problem, because the apparatus only sends the first message, after I send back the acknowledgement, it doesn't respond anymore.
Here is the code

private void listener()       {
       try    {
            // ... variables and stuff...

            // Enter the listening loop.
            while (!done)  {
                // Perform a blocking call to accept requests.
                TcpClient client = tcpListener.AcceptTcpClient();

                // Get a stream object for reading and writing
                stream = client.GetStream();
                lanReader = new BinaryReader(stream);
                lanWriter = new BinaryWriter(stream);

                conversation = true; //will be set to false again after END
                string cache = "";
                // ... (this part works)
                processReceivedData();
                // ...
           }
     }
}
// after receiving data in a loop above, it's being processed by the following function:

private void processReceivedData()       {
     if (daten.Contains("<HEL.") && daten.Contains("</HEL")) { // Hello Message received
          sendACK();
     }
     // ... (status, other messages, measurements...)
}


private bool sendACK()    {
    if (lanWriter.BaseStream == null) //lanWriter initialized in the beginning
         return false;
    lanWriter.Write(ACK); //ACK: Acknowledgement message string
    return true;
}

So basically, after the apparatus starts communication, the reading and writing streams are opened, but simply writing in it doesn't seem to work. What am I doing wrong?
Do I have to set the IP of the apparatus somewhere (I can access that)?