HttpWebRequest 获取源文件

下面代码是我获取源文件的代码，但是最近出了问题，我是用来查找搜搜的排名但是现在 只能获取到一部分网页的代码，不知道为什么  private static string GetHtml(string url, string charSet, bool ck)
        {
            try
            {
                var request = (HttpWebRequest)WebRequest.Create(url);
                request.Timeout = 0x4e20;
                request.Accept = "*/*";
                request.UserAgent =
                    "Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0; .NET CLR 3.5.21022;)";
                //request.UserAgent = "Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.2; .NET CLR 1.1.4322; .NET CLR 2.0.50727; InfoPath.1) Web-Sniffer/1.0.24";
                request.Headers.Add("Accept-Language", "zh-cn");
                if (ck)
                {
                    request.Headers.Add("Cookie",
                                        "PREF=ID=5927b7eb73235d69:U=03fa9a3bc2ac3a36:FF=1:LD=zh-CN:NW=1:S=51bSbtuCgB-HmdQL; NID=49=GhtmWYVJPrZzT007RVs6jYKmSjXJfqfQurc5NkEgKGh4l0j8o1MfSyW1iPu2k0v6hMqe7-y4YssanTTZ4w69THlRArq2rSN62BR_flUU54LgZoou-RdCfQZMyuA8qxGL");
                }
                Stream responseStream = request.GetResponse().GetResponseStream();
                if (responseStream != null) responseStream.ReadTimeout = 0x3a98;
                if (charSet == "")
                    charSet = "gb2312";
                if (responseStream != null)
                {
                    var reader = new StreamReader(responseStream, Encoding.GetEncoding(charSet));
                    return reader.ReadToEnd();
                }
            }
            catch (Exception)
            {                return "";
            }
            return null;
        }