session time out automatically in asp.net web application

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C#3.5

Hi ,
i m developing web application in asp.net with c#.I m Creating session in login page.
and displaying it to another page.it works fine,but when i upload it to server.
After 20 -30 seconds my session is automatically timed out. i m using web.config, in which session timeout is 20 minute.
can anyone solve my problem
thanks in advance.

My code is shown as below:
in login page button click event is as follows:

string usertype;

protected void btnlogin_Click(object sender, EventArgs e)
    {
        cmd = new SqlCommand("Select UserName,Password,Type from tblLogin", con);
        dr = cmd.ExecuteReader();
        while (dr.Read())
        {
            if (txtusername.Value == dr[0].ToString() && txtpass.Value == dr[1].ToString())
            {
                usertype = dr[2].ToString();
                flag = true;
                break;
            }
            else
            {
                flag = false;
            }
        }
        dr.Close();
        if (flag == true)
        {
            Session["utype"] = usertype;
            Response.Redirect("Default3.aspx");
        }
        else
        {
            //Response.Redirect("~/GPS/Default.aspx");
            //Response.Write("Username " + dr[0].ToString());
            //Response.Write("password " + dr[1].ToString());
            //Response.Write("Invalid LoginName Or Password");
            Response.Redirect("Default.aspx");
        }
    }

and i print this value in another page like this on page load event.

string sessname;
    protected void Page_Load(object sender, EventArgs e)
    {
        sessname = Session["utype"].ToString();
        Response.Write(sessname);
    }