Session values get null after page get post-back ?

protected void Page_Load(object sender, EventArgs e)
     {
          if(!Page.IsPostBack)
{
	Session["EmployeeName"]="Djay";
}
}

   protected void ddlProcess_SelectedIndexChanged(object sender, EventArgs e)
        {
            try
            {
               if(Session["EmployeeName"] !=null)
                {
                     // some code here
                }
                else
               {
                 // every time Debugger comes here after page post back
                }
            }
            catch (Exception)
            {
                Response.Redirect("/SitePages/SessionExpire.aspx", false);
            }
        }

What I have tried:

1. I used windows authentication (can't change to form authentication it's required )
2. and set <sessionstate mode="InProc" customprovider="DefaultSessionProvider" timeout="60"> in web config still face issue.</sessionstate>