Expectation failed exception during post using HttpWebRequest

I'm using this code in android to get a response from a server using POST:

public  string PostDataRequest(string url,string postData){
            WebResponse response = null;
            Stream dataStream = null;
            StreamReader reader = null;
            try {
                    HttpWebRequest httpWReq =
                    (HttpWebRequest)WebRequest.Create(url);
                    ASCIIEncoding encoding = new ASCIIEncoding();
                    byte[] data = encoding.GetBytes(postData);
                    httpWReq.Method = "POST";
                    httpWReq .KeepAlive = true ;
                    httpWReq.ContentType = "application/x-www-form-urlencoded";
                    httpWReq.ContentLength = data.Length;

                    using (Stream newStream = httpWReq.GetRequestStream())
                    {
                        newStream.Write(data, 0, data.Length);
                    }
                    response = httpWReq.GetResponse();
                    // Get the stream containing content returned by the server.
                    dataStream = response.GetResponseStream();
                    // Open the stream using a StreamReader for easy access.
                    reader = new StreamReader(dataStream);
                    // Read the content.
                    string responseFromServer = reader.ReadToEnd();
                    // Clean up the streams.
                    reader.Close();
                    dataStream.Close();
                    response.Close();
                    //return the content.
                    return responseFromServer;

            } catch (Exception ex) {
            }

I encounter "Expectation failed" exception when I run the app , but when I use hotspotshield on my phone it works fine!when I use the url in a browser it works fine even without hotspotshield. why is that and how can I fix this issue. most of forums suggest this line of code : System.Net.ServicePointManager.Expect100Continue = false; but it didn't work. please advise. Thank you