GZipStream not send when I want

I'm trying to use GzipStream over a socket. For this purpose I use something like this.

using System.IO.Compression;    
TcpClient _client = null;

// ...
// ... Connection 

NetworkStream networkStream = _client.GetStream();
GZipStream zip = new GZipStream(networkStream, CompressionMode.Compress);

when I send a message I use GZipStream methods, Write and Flush:

zip.Write (....);
zip.Flush();

The problem is that when I run Flush method the message isn't sent. The messages are only sent when I close GZipStream (running Close method of GZipStream class).

I'm trying with this approach:

using (GZipStream zip = new GZipStream(networkStream, CompressionMode.Compress))
{
    zip.Write (....);
}

but, when zip class is out of using scope and is disposed (closed), networkStream instance is closed and disposed too.

How can avoid this behaviour? I'm running this in a realtime application and I need to send messages when I want not when GZipStream wants.

Any ideas?

Thank you.

UPDATE:

I have create a complete snipet.

using (var client = new TcpClient("127.0.0.1", 1514))
{
    var networkStream = client.GetStream();
    client.SendBufferSize = 1;
    client.NoDelay = true;

    using (Stream zip = new GZipStream(networkStream, CompressionMode.Compress, true))
    {
        byte[] dgram = CreateMessage("Testing line 1");
        zip.Write(dgram, 0, dgram.Length);     
    }
    networkStream.Flush();

    // Next using block don't send anything. The server side does't receive anything.  
    // No exception is raised. It seems ok, but anything is sent
    using (Stream zip = new GZipStream(networkStream, CompressionMode.Compress, true))
    {
        byte[] dgram = CreateMessage("Testing line 2");
        {
            zip.Write(dgram, 0, dgram.Length);
        }
    }
    networkStream.Flush();
}