In my case, I had overridden the default Forms .cshtml files, and the issue was the following:
The class umbraco-forms-form was missing from the div which wraps the form (the div with id="umbraco_form_xx”)
The class `umbraco-forms-page` was missing from the individual page (the form is multi page)
I was missing some lines in Scripts.cshtml
I had a field which was more of an information screen. This required adding a hidden field with the data-umb attribute to trigger the conditional script