ModalPopupExtender for a n panel in aspx page is not disabling the controls in the background

<asp:Panel ID="AddGroupPopup" runat="server" CssClass="ModelPopUp" > ------------------------------- <ajaxToolkit:ModalPopupExtender ID="ModalPopupExtenderAddGroup" runat="server" TargetControlID="btnGroup" BehaviorID="ModalPopupBehaviorAddGroup" PopupControlID="AddGroupPopup" BackgroundCssClass="modalBackground" DropShadow="true" CancelControlID="btnGRCancel"> </ajaxToolkit:ModalPopupExtender> --- ...