Modal Popup Extender Button - Not Firing On Click

Question

I cannot see what I've got setup wrong here, appreciate the help.

 <asp:HiddenField ID="hfFakeDownload" runat="server" />
 <ajaxToolkit:ModalPopupExtender ID="mpeDownload" runat="server" TargetControlID="hfFakeDownload" PopupControlID="pnlDownload" BackgroundCssClass="modalBackground" />


<asp:Panel runat="server" ID="pnlDownload" CssClass="div" Style="display: none; width: 350px;">
<asp:LinkButton ID="lnkCloseRule" runat="server" Text="Close" Style="float: right;" OnClick="CloseButton_Click" />
<div class="clear-fix"></div>

<asp:Panel runat="server" ID="pnlDownloadInner" CssClass="div">
    <h3 style="margin-top: 0">Test</h3>

    <table class="center" style="vertical-align: bottom">

        <tr>
            <td style="text-align: left;">
                <asp:DropDownList ID="ddlDownloadFiles" runat="server" Width="200px" />
            </td>
            <td>
                <asp:Button ID="DownLoadButton" runat="server" Text="Download" OnClick="DownLoadButton_Click" />
            </td>
        </tr>
    </table>
</asp:Panel>

</asp:Panel>

Code Behind

protected void DownLoadButton_Click(object sender, EventArgs e)
    {
        long taskId = Convert.ToInt64(lblTaskIdDownload.Text);

        DownloadFile("Task", taskId, downloadFileName);
    }

From what I can see this all looks correct, just not postback coming on the page.

Thanks.