How to update info in popup window from the Parent window with session values?

Greeting people,
I have two web forms, the parent (WebForm1.aspx) and the popup (WebForm2.aspx). I use session values to pass data from the parent (Weborm1) to the Popup (WebForm2). The info on Parent is not always passed over to the Popup. If I use QueryString, it works fine, but not with session. Below is the code (VS2008 C#):
The Parent (Webform1.aspx):
Two TextBoxes (txtUserName and txtUserRegion), and one button (btnOpenPopup)
 The btnOpenPopup click event:
protected void btnOpenPopup_Click(object sender, EventArgs e)
{
    Session["UserName"] = txtUserName.Text.ToUpper().Trim();
    Session["UserRegion"] = txtUserRegion.Text.ToUpper().Trim();

    Page.ClientScript.RegisterStartupScript(this.GetType(), @"startup", @"<script>window.open('WebForm2.aspx','myWindow', 'toolbar=yes,location=yes,statusba ...