Session gets null in masterpage

upon login I Already pass the parameter in session:
then the use of that session nickname was design on my Masterpage part like "Welcome Mary Jean" on a header part:

login page:
var login = (from usr in context.Dim_Users
  where usr.LoginName == txtUserName.Text.ToUpper().Replace(@"PCPPI\", "")
        && usr.Active == true
  && usr.Plant_PIdx != null && usr.Dept_PIdx != null
  select usr).ToList();

  if (login.Count > 0)
  {
 
  Session["User_Idx"] = login[0].User_Idx;
  Session["NickName"] = login[0].NickName;
  Session["Admin"] = login[0].Admin;
  Session["Dept"] = login[0].Dept_PIdx;
   Session["Plant"] = login[0].Plant_PIdx;
  Session["UserName"] = login[0].LoginName;
  successfullylogin = true;
  }

on my MasterPage: I called that session
protected void Page_Load(object sender, EventArgs e)
{
        if (!IsPostBack)
        {
                lnkUser.Text = Session["NickName"].ToString();



        }

{

can you paste the code where you assign the value to Session["NickName"] ?

in this file there is no error ,,,i think you are doing mistake on another area,,,

So Please check these points...

1)  somewhere on any content page are you making null to NickName session.
2.) first content page's page load event fire then master page's load event fire.
3.) are you assigning proper value in NickName session.