C＃WebClient在第二个请求时响应超时

Question

I am creating a C# Application that create a WebClient request to a PHP file on server and perform some operation and WebClient request is in loop but i am facing a problem that WebClient request on first iteration works as needed but second call gives timeout. I am not sure weather some value is missing in WebClient request or to refresh the call. Code is below.

using (WebClient client = new WebClient()){
    var values = new NameValueCollection();
    values["db_op_and_remove_file"] = "1";
    values["path"] = file_path;
    values["question"] = question;
    values["product_id"] = product_id;
    values["tempdir"] = tempdir;
    client.Headers[HttpRequestHeader.ContentType] = "application/x-www-form-urlencoded";
    client.UseDefaultCredentials = true;
    var response = client.UploadValues(server_header, values);
    responseString_keyword = Encoding.Default.GetString(response);
}