Modal PopUpExtender button doesn't fire back to the server

Hi,

Any idea why clicking the "ButtonCancelOK" button in the following code doesn't fire back to the server?

<asp:Panel ID="cancelConfirmation" align="center" CssClass="ModalWindow" style = "display:none" runat="server"><div class="popup_header">
                            Validation Message</div><div class="popup_Body">
                            Are you sure you want to CANCEL the changes?</div><div class="popup_Buttons"><asp:Button ID="ButtonCancelOkay" runat="server" Text="Okay" onclick="ValidationOK_Click"/><asp:Button ID="ButtonCancelCancel" runat="server" Text="Cancel" /></div></asp:Panel><ajaxToolkit:ModalPopupExtender ID="ModalPopupExtender1" runat="server" 
                        TargetControlID="cancel_edit" OkControlID="ButtonCancelOkay" CancelControlID="ButtonCancelCancel" 
PopupControlID="cancelConfirmation" DropShadow="true" BackgroundCssClass="modalBackground"><Animations><OnShown><FadeIn Duration=".4" Fps="20" />                </OnShown></Animations></ajaxToolkit:ModalPopupExtender>