ASP.NET 2.0 required field validator is not working on production server

when I use a REQUIRED FIELD VALIDATOR on a page it works fine on my machine (Development Mechine), but when I deploy the pages to production Server the Validator does not work.

This problem occurs across the application. The development mechine and producton machine are running under Windows 2003 server (service pack-2) operating system.


UI-Code
----------
<asp:TextBox runat="server" ID="txtTelephoneNumber" CssClass="Ms-long" MaxLength="40"
                                    Width="150" TabIndex="3">
 </asp:TextBox>
 <asp:RequiredFieldValidator runat="server" ErrorMessage="Please enter Telephone number"
                                    ID="reqTelephone" Display="Dynamic" ControlToValidate="txtTelephoneNumber">
</asp:RequiredFieldValidator>

<asp:Button runat="server" Text="OK" ID="btnSave"  OnClick="btnSave_Click" CssClass="Ms-ButtonHeightWidth"    TabIndex="4" />

I have copied the aspnet_client folder from development system to production server then also is not working.