Htmlagilitypack doc.loadhtml can't get whole HTML string

/// <summary>
/// http://www.codeproject.com/Articles/18034/HttpWebRequest-Response-in-a-Nutshell-Part
/// </summary>
/// <param name="contents_url">The URL string.</param>
private static void get_cotents(string contents_url)
{
    byte[] buffer = new byte[1024];
    HttpWebRequest WebReq = (HttpWebRequest)WebRequest.Create(contents_url);
    WebReq.Method = "POST";
    WebReq.ContentType = "application/x-www-form-urlencoded";
    WebReq.ContentLength = buffer.Length;
    Stream PostData = WebReq.GetRequestStream();
    //Now we write, and afterwards, we close. Closing is always important!
    PostData.Write(buffer, 0, buffer.Length);
    PostData.Close();
    //Get the response handle, we have no true response yet!
    HttpWebResponse WebResp = (HttpWebResponse)WebReq.GetResponse();

    //Let's show some information about the response
    Console.WriteLine(WebResp.StatusCode);
    Console.WriteLine(WebResp.Server);

    //Now, we read the response (the string), and output it.
    Stream datastream = WebResp.GetResponseStream();
    StreamReader answer = new StreamReader(datastream);
    Console.WriteLine(answer.ReadToEnd());
    datastream.Close();
    answer.Close();
}