SPLongOperation not showing modal

I have the following code:

protected void Sync_Btn_Click(object sender, EventArgs e)
        {
            try{
                using (SPLongOperation SyncDBLo = new SPLongOperation(this.Page))
                {
                    SyncDBLo.LeadingHTML = "Running DB Sync Now";
                    SyncDBLo.TrailingHTML = "This may take a few minutes";
                    SyncDBLo.Begin();
                    Begin_Sync(); //LONG RUNNING CODE
                    EndLongOp(SyncDBLo);
            }
            catch (System.Threading.ThreadAbortException) { }
            catch (Exception ex) { Microsoft.SharePoint.Utilities.SPUtility.TransferToErrorPage(ex.ToString()); }

        }

        protected void EndLongOp(SPLongOperation operation)
        {
            HttpContext context = HttpContext.Current;
            if (context.Request.QueryString["IsDlg"] != null)
            {
                context.Response.Write("<script type='text/javascript'>window.frameElement.commitPopup();</script>");
                context.Response.Flush();
                context.Response.End();
            }
            else
            {
                string url = SPContext.Current.Web.Url;
                operation.End(url, Microsoft.SharePoint.Utilities.SPRedirectFlags.CheckUrl, context, string.Empty);
            }
        }

I got most of this from other sources indicating it would work.  It seems to run as my page doesn't time out, but no modal dialog is ever displayed.  I wanted to get that working as I've found other code that shows how I may be able to update the modal with information as the process runs but I need to get the modal first.

Please let me know if anyone knows why the dialog is not appearing.

Thank you!