To clarify, are you expecting the non-HubSpot forms functionality to automatically detect your form submission, or are you making HTTP requests to the Forms API?
If you are using non-HubSpot forms, ensure that you are submitting your form on an unfiltered IP address (e.g. your mobile device while using cellular data or Opera with VPN mode enabled) and that your form meets all of the criteria specified in this documentation. Common issues include:
The email field is not an email type input, as in: <input type="email">.
The form isn't using a standard <input type="submit"> button for its form submission. A generic HTML <button> (or other element) will prevent HubSpot from capturing submissions.
The form has JavaScript bound to the form submit event or submit button click event. HubSpot also uses an event listener so other custom event listeners can prevent HubSpot from capturing submissions.