ValidatorCalloutExtender is not visible

[dotnetprodev]

I have downloaded and ran the Ajax Toolkit samples. They do work. For
some reason mine does not.

This is the code for my login page. I do not get any errors; it simply
does not show the callout.

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<%@ Page Language="VB" AutoEventWireup="false" CodeFile="login.aspx.vb"
Inherits="login" MasterPageFile="~/default.master" %>
<%@ Register Assembly="System.Web.Extensions, Version=1.0.61025.0,
Culture=neutral, PublicKeyToken=31bf3856ad364e35"
Namespace="System.Web.UI" TagPrefix="asp" %>
<%@ Register Assembly="AjaxControlToolkit"
Namespace="AjaxControlToolkit" TagPrefix="cc1" %>

<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1"
Runat="Server">
<asp:ScriptManager ID="scriptManager" runat="server" />
<table id="login_form">
<tr>
<td colspan="2" class="header">Log In</td>
</tr>
<tr>
<td class="field_header">User Name:</td>
<td><asp:TextBox ID="txtUserName" runat="server" />
<asp:RequiredFieldValidator ID="rfvUserName"
ControlToValidate="txtUserName" Display="None" runat="server"
ErrorMessage="Please enter a UserName.<br>If you cannot remember your
UserName click <a href='forgot_username.aspx'>here</a>." />
<cc1:ValidatorCalloutExtender
ID="vceUserName"
TargetControlID="rfvUserName"
CloseImageUrl="~/images/close.gif"
WarningIconImageUrl="~/images/alert-large.gif"
HighlightCssClass="highlight"
runat="server">
</cc1:ValidatorCalloutExtender>
</td>
</tr>
<tr>
<td class="field_header">Password</td>
<td><asp:TextBox ID="txtPassword" TextMode="Password"
runat="server" />
<asp:RequiredFieldValidator ID="rfvPassword"
ControlToValidate="txtPassword" runat="server" ErrorMessage="*" /></td>
</tr>
<tr>
<td colspan="2"><asp:CheckBox ID="cbRememberMe"
Text="Yes, remember me next time I visit." runat="server" /></td>
</tr>
<tr>
<td colspan="2"><asp:Label ID="lblLoginStatus"
ForeColor="Red" runat="server" /></td>
</tr>
<tr>
<td colspan="2"><asp:Button ID="btnLogin"
runat="server" Text="Log In" CssClass="login_button" /></td>
</tr>
<tr>
<td colspan="2">
<asp:HyperLink ID="hlNeedAccount"
NavigateUrl="~/create_user.aspx" Text="Need an Account?" runat="server"
/><br />
<asp:HyperLink ID="hlForgotPassword"
NavigateUrl="~/forgot_password.aspx" Text="Forgot Password?"
runat="server" />
</td>
</tr>
</table>
</asp:Content>

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