Button click events take time to perform in ASP.Net

When trying to capture and convert HTML DIV to image, the button click event does not fire immediately, I always have to click the button twice or more in order for it to work.

May I please ask why button click does not fire immediately after it is clicked, in this regard?

How can this be corrected please?

HTML

<asp:Label ID="LabelName" runat="server" Text="Richard"></asp:Label>
<asp:Image Class="imgfile" ID="imgfile" runat="server" />
<br />
<asp:Button ID="SaveDoc" runat="server" CssClass="btn btn-primary navbar-btn" Text="Save Document"
    OnClick="DownloadDocument_Click" UseSubmitBehavior="false" OnClientClick="return ConvertDocument(this)" />
<asp:HiddenField ID="ImageDatahf" runat="server" />
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<script type="text/javascript" src="https://html2canvas.hertzen.com/dist/html2canvas.min.js"></script>
<script type="text/javascript">
    function ConvertDocument(SaveDoc) {
        html2canvas($("#section")[0], { scale: 3 })
        .then(function (canvas) {
            var base64 = canvas.toDataURL();
            $("[id*=ImageDatahf]").val(base64);
            __doPostBack(SaveDoc.name, "");
        });
    }
</script>

Code

protected void DownloadDocument_Click(object sender, EventArgs e)
{
    try
    {
        using (SqlConnection con = new SqlConnection())
        {
            con.ConnectionString = ConfigurationManager.ConnectionStrings["ConString"].ConnectionString;

            string base64 = Request.Form[ImageDatahf.UniqueID].Split(',')[1];
            byte[] imageBytes = Convert.FromBase64String(base64);
            iTextSharp.text.Image image = iTextSharp.text.Image.GetInstance(imageBytes);
            using (System.IO.MemoryStream memoryStream = new System.IO.MemoryStream())
            {
                if (Convert.ToInt64(image.Width.ToString()) > Convert.ToInt64(image.Height.ToString()))
                {
                    using (Document document = new Document(new RectangleReadOnly(842, 595), 0f, 0f, 0f, 0f))
                    {
                        PdfWriter writer = PdfWriter.GetInstance(document, memoryStream);
                        document.Open();
                        image.SetDpi(100, 100);
                        image.ScaleToFit(500f, 1000f);
                        document.Add(image);
                        document.Close();
                    }
                }
                else
                {
                    using (Document document = new Document(new RectangleReadOnly(595, 842), 0f, 0f, 0f, 0f))
                    {
                        PdfWriter writer = PdfWriter.GetInstance(document, memoryStream);
                        document.Open();
                        image.SetDpi(100, 100);
                        image.ScaleToFit(500f, 1000f);
                        document.Add(image);
                        document.Close();
                    }
                }
                byte[] bytes = memoryStream.ToArray();
                memoryStream.Close();
                // Code for saving binary data in daabase.
                using (SqlCommand cmd = new SqlCommand("INSERT INTO DocumentTable (filename, ContentType, Data, Recipient)" + "VALUES(@filename, @ContentType, @Data, @Recipient)", con))
                {
                    cmd.Parameters.AddWithValue("@filename", "Certificate.pdf");
                    cmd.Parameters.AddWithValue("@ContentType", "application/pdf");
                    cmd.Parameters.AddWithValue("@Data", bytes);
                    cmd.Parameters.AddWithValue("@Recipient", LabelName.Text.Trim());
                    con.Open();
                    cmd.ExecuteNonQuery();
                    con.Close();
                    Response.Redirect(Request.Url.AbsoluteUri);
                }
            }
        }
    }
    catch (SqlException ex)
    {
        string msg = "Error:";
        msg += ex.Message;
        throw new Exception(msg);
    }
}