Button function no trigger but modalpopup is display properly...

Hi guys.. May i know why btn_request_tac_Click no trigger? (BUT modalpopup is display properly)

my codebehind..

Sub btn_request_tac_Click(ByVal sender As Object, ByVal e As System.EventArgs)
        Me.lbl_message.Text = "Successful"
End Sub

<asp:LinkButton ID="btn_request_tac" runat="server" onclick="btn_request_tac_Click">Request a TAC number</asp:LinkButton><cc1:ModalPopupExtender ID="mpe1" runat="server" PopupControlID="panel1" TargetControlID="btn_request_tac" CancelControlID="btnCancel"></cc1:ModalPopupExtender><asp:Panel ID="panel1" runat="server"><table><tr><td><asp:Label ID="lbl_message" runat="server" Text=""></asp:Label></td></tr></asp:Panel>