If a WP Simple Pay form isn’t initialized properly and there are no errors in the console, it is usually a decent indicator that nothing has happened, which is usually a good indicator that nothing has loaded.
You can view the page source, and if none of the WP Simple Pay scripts nor any other JavaScript are output in the footer, it usually means that their theme, or that page on their site, isn’t using: <?php wp_footer(); ?>.
This will need to be corrected for WP Simple Pay to work properly.