Some common reasons why HubSpot would not be able to capture submissions on Non-Hubspot forms are:
Form is not enclosed in<form>tags.
Form is contained inside<iframe> tags.
Form has JavaScript bound to the form submit event or submit button click event. This is how the tool captures submissions and any other events, and can prevent HubSpot from knowing when submissions occur.
Form was introduced after the page completed loading. The tool scans the page as soon as it loads and listens to submissions on forms that were present at that point.
Form is on a single-page app. In most single-page apps, the HTML is not loaded when navigating to another page. As a result, HubSpot cannot capture these form submissions.
Form contains a sensitive field, such as credit card number.
Form contains hidden fields. The tool does not collect information from any hidden fields.
The email field is not an email type input, as in: <input type="email">.
Form is not using a standard <input type="submit"> button for its form submission. The formmusthave a standard <input type="submit"> button to capture submissions.
Please check if all of the above are marked correct. Also, please make sure that "Collect data from website form" toggle is on. You can check this here Settings > Marketing > Forms > Non HubSpot Forms.
I hope that helps, but I'm open to talk through this further with you! If you think my answer is a possible solution to your query, please mark it as a Solution.