The label text doesn't show up in modalpopupextender , assigned from vb code behind

Here is the code snippet in aspx page for modalpopupextender:

<cc1:ModalPopupExtender ID="mpApproveAlert" runat="server" PopupControlID="Panel1" TargetControlID="btnShow"
CancelControlID="btnClose" BackgroundCssClass="modalBackground"></cc1:ModalPopupExtender><asp:Panel ID="Panel1" runat="server" CssClass="modalPopup" align="center" style = "display:none"><table width="100%" cellpadding="0" cellspacing="0"><tr><td class="modalPopup_text"><asp:Label ID="lblmodalMsg" runat="server" CssClass="modalLabel"   ></asp:Label></td></tr><tr><td class="modalPopup_text"><asp:Button ID="btnSignOut" runat="server" Text="Ok" CssClass="btn btn-primary modal_ok" UseSubmitBehavior="false"></asp:Button></td><td><asp:Button ID="btnClose" runat="server" CssClass="btn btn-success modalPopup_close" Text="x" /></td></tr></table></asp:Panel>

This modal popup is displayed on click of a button:

Here is the code snippet from where modalpopup label is assigned a text:

...some code here...
           lblmodalMsg.Text ="Reservation has been approved successfully!"
            mpApproveAlert.Show()Else
            lblmodalMsg.Text ="No data found to approve!"
            lblmodalMsg.Visible =True
            mpApproveAlert.Show()EndIfEndSub

I have tried using updatepanel instead of panel, but of no use.