Why doesn't asp.net button work with modal popup?

Question

I have used an AJAX MODAL POPUP in asp.net page inside ajax update panel.

It works when I call it from gridview i.e. rowcommand event via link button.

mdlAuthorizedEntities.Show();

but same modal doesn't work when called via asp button outside gridview i.e.

 <asp:LinkButton ID="ImgbtnAddMoreEntities" Visible="true" tyle="display:none;"  CssClass="plus" Text="add" OnClick="ImgbtnAddMoreEntities_Click" runat="server" />

.cs:

   protected void ImgbtnAddMoreEntities_Click(object sender, EventArgs e)
    {
        try
        {
            mdlAuthorizedEntities.Show();
        }
        catch (Exception ex)
        {
            ResultLabel.ResultLabelAttributes(ex.Message, ProjectUserControls.Enums.ResultLabel_Color.Red);
        }
        finally
        {
            ResultPanel.Controls.Add(ResultLabel);
        }
    }

.modal