CalendarExtender initially appears blank

I have a small standalone development project and am designing a form page in it.

It has a CalendarExtender, I notice that when it is initially popped up it is blank appart from the left/right arrow.

Pressing an arrow and the month appears OK and from then on it's OK.

Any ideas why.

If I create a brand new project and paste the code in it then works OK.

So it must be something in my project I think.

Jim ( Code below )

<%@ Page Title="" Language="vb" AutoEventWireup="false" MasterPageFile="~/Site1.Master"
    CodeBehind="JimsTestPage.aspx.vb" Inherits="BiflDemo.JimsTestPage" %><%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %><asp:Content ID="Content2" ContentPlaceHolderID="DetailsPage" runat="server"><cc1:ToolkitScriptManager ID="ScriptManager2" runat="server" EnableHistory="True"></cc1:ToolkitScriptManager><asp:UpdatePanel ID="UpdatePanel1" runat="server"><ContentTemplate><tr><td class="col2"><asp:TextBox ID="txtFromDate" runat="server"
                        AutoPostBack="True" TabIndex="5"></asp:TextBox><cc1:CalendarExtender ID="txtFromDate_CalendarExtender" runat="server" TargetControlID="txtFromDate"></cc1:CalendarExtender></td></tr></ContentTemplate></asp:UpdatePanel></asp:Content>