Page_load Only occurs Once

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ASP.NET

I would like my application to always call the page_load when loading this page

SearchBillType.aspx

Imports System.Data
Imports System.IO
Imports System.Xml
Partial Class SearchDiagnosis
    Inherits PageBase
    Implements IPostBackEventHandler
    Shadows IsCallBack As Boolean = False
    Dim MAFunctions As New MAFunctions

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
     LoadBillType()     If IsPostBack AndAlso Request.Form("__AsyncPost") IsNot Nothing AndAlso Request.Form("__AsyncPost") = "true" Then
         IsCallBack = True
     End If
     If Not IsPostBack And Not IsCallBack Then
         If Not Session("BillTypeSelected") Is Nothing Then
             Session.Remove("BillTypeSelected")
         End If
     End If
 End Sub

This code only gets called the first time my application loads this page.

The following is a snipet from my Main page showing the Javascript code which calls the SearchBillType Page

<script type="text/javascript">
     var clientid = "";
     var clientname = "";
     function ShowModal(Mode, Width, Height) {
         var browserName = navigator.appName;
         switch (Mode.toLowerCase()) {
             case "billtype":
                 var modalResult = window.showModalDialog(
                        "SearchBillType.aspx",
                        "resizable:no;scroll:yes;status:no;",
                        "dialogWidth:" + Width + ";dialogHeight:" + Height + ";center:yes"
                         );
                 document.getElementById("<%=btnParentChild.ClientId%>").click();
                 break;
            }
     }

Thank you.