How I keeping login with httpwebrequste?

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C#

.NET

i try login to website using HttpWepRequste and i sucuss login
the problem when i try go on other url then ask me to log in again
and thats my code where is the problem ???
and thats my code

What I have tried:

CookieContainer cook = new CookieContainer();
            string data = "act=login&accesskey=&si_user=a7mad-bas%40hotmail.com&si_pass=1236987412&si_stay=1&x=14&y=32";
            Encoding enc = new UTF8Encoding();
            byte[] postdata = enc.GetBytes(data);

            HttpWebRequest req = (HttpWebRequest)WebRequest.Create("http://[site].com/en/account/");
            
            req.Method = "GET";
            req.KeepAlive = true;
            req.UserAgent = "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:43.0) Gecko/20100101 Firefox/43.0";
            req.ContentType = "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8";
             req.CookieContainer=cook ;
            logincook = cook;

            HttpWebResponse res = (HttpWebResponse)req.GetResponse();
            foreach (Cookie c in res.Cookies)
            {
                cook.Add(c);
            }
            logincook = cook;
            

            StreamReader sr = new StreamReader(res.GetResponseStream());
            string x = sr.ReadToEnd();
            richTextBox1.Text = x;
            //res.Close();

            //post Method 

            req = WebRequest.Create("http://[site].com/en/account/") as HttpWebRequest;
            
            req.Method = "POST";
            req.KeepAlive = true;
            req.UserAgent = "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:43.0) Gecko/20100101 Firefox/43.0";
            req.ContentType = "application/x-www-form-urlencoded";
            req.ContentLength = postdata.Length;
            cook= req.CookieContainer  ;
           
            logincook = cook;
            Stream wr = req.GetRequestStream();
            wr.Write(postdata, 0, postdata.Length);

            res = req.GetResponse() as HttpWebResponse;
            foreach (Cookie c in res.Cookies)
            {
                cook.Add(res.Cookies);
            }
            logincook = cook;

            sr = new StreamReader( res.GetResponseStream());
            x = sr.ReadToEnd();
            richTextBox2.Text = x;

            //MessageBox.Show(logincook.Count.ToString());
            req = WebRequest.Create("http://[site].com/en/account/") as HttpWebRequest;
            
            req.Method = "GET";
            req.KeepAlive = true;
            req.UserAgent = "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:43.0) Gecko/20100101 Firefox/43.0";
            req.ContentType = "application/x-www-form-urlencoded";
           // req.ContentLength = postdata.Length;
            req.CookieContainer = cook;
            logincook = cook;