Some page builders (like Thrive Architect) may change the behavior of the SpeakOut! shortcode so that the CSS header file is not loaded. For example:
[emailpetition id="1"] To check for this, look at the page source in question and you should see:
<head>
...
<link rel='stylesheet' id='dk_speakout_css-css' href='https://$YOUR_SITE_FQDN/wp-content/plugins/speakout/css/theme-default.css?ver=$LATEST_SPEAKOUT_VERSION' media='all' />
...
</head>
If you don't see this, then SpeakOut! has no way of communicating with the wordpress backend. The easiest way to fix this, is to ensure the CSS file is added to the header file. With WordPress there a number of plugins that will allow to inject custom scripts into the header of your page. Pick one and make sure the CSS file is loaded.