Session variable null in Asp.net MVC 4 and AngularJS web app

Question

On user sign in I'm calling the below sign in controller using angularjs $http post for sign in.

public JsonResult SignIn(string username, string password)
{
    password = PasswordEncryptionDecryption.Encrypt(password);

    User user = new User() { Username = username, Password = password };
    userBL = new UserBL(user);
    User response = userBL.ValidateUserLogin();
    response.ProfileImageUrl = ConfigurationManager.AppSettings["Profile"] + response.ProfileImageUrl;
    SessionManager.SetLoggedinuser(response);

    Session["LoggedInUser"] = response.PrimaryId;
    Session["LoggedInUserRole"] = response.UserRole;
    Session["LoggedInUserName"] = response.Name;
    Session["LoggedInProfileImageUrl"] = response.ProfileImageUrl;

    return Json(new { response = response, redirectUrl = "Thank/Index" });
}

The above controller sets the Session variables and returns JSON result along with the redirect url. I'm redirecting using JavaScript from client side. Here is the code:

$scope.SignIn = function () {
    $http({
        // set the parameter for request
        method: 'POST',
        url: 'Signin/SignIn',
        headers: {
            'Content-Type': 'application/json; charset=utf-8'
        },
        data: { username: $scope.user.username, password: $scope.user.password }
    })
    .then(function (resp) {
        //
        console.log(resp);
        if (resp.data.response.StatusCode == 200) {
            $window.location.href = rootUrl + resp.data.redirectUrl;
        } else {
            alert(resp.data.response.StatusMessage);
        }
    },
    function (error) {
        //
    });
});

I'm using the Session["LoggedInUserName"] in my _Layout.cshtml to display the logged in user name.

Here is the Thank Controller code :

public class ThankController : Controller
{
    private ThankYouNotes thankYouModel;
    private ThankYouBL thankYouBL;

    public ActionResult Index()
    {
        return View();
    }

    public JsonResult GetThankYouNotes()
    {
        thankYouBL = new ThankYouBL();
        List<ThankYouNotes> thankYouList = thankYouBL.GetThankYouNotes(SessionManager.CurrentLoggedinuser.PrimaryId);
        return Json(new { response = thankYouList });
    }
}

The issue is that on page refresh the Session becomes null and if I refresh further it gets set. I really can't figure what I'm doing wrong.