If the email input field on your webpage is embedded using an iframe, then you'll need to follow these steps to capture conversions.
How to fix
Right click on the area of the page being generate by the iframe > select 'View Frame Source'. You may need to click around a few times until you find the area of the page that contains the iFrame.
Copy the URL from your browser's address bar. Make sure NOT to include the question mark and anything characters after (commonly referred to as the query string).
Paste the url into the Capture section of your Proof campaign. Please make sure that the pixel is also installed on this new capture url.