Possible cause
If the form won't submit when you click the submit button, it may be because a required field is hidden. Hidden items are invisible on screen, but the elements themselves still exist.
How to check (1): Look for alerts in the right panel
Select the form.
Open the [Form] tab in the right panel. If the right panel is closed, open it first.
Look for the alert: [A required field is hidden, so the form cannot be submitted.] Any box marked with [!] is hidden at a specific breakpoint.
How to check (2): Review the Layers panel
Open the Layers panel for the form and check whether the box for any required field is set to hidden at any breakpoint.
How to check (3): Check the browser console log
Open the browser console log on the live preview or published site.
If a required field hasn't been filled out, you'll see an alert like the one below. (Example: the image below shows an alert for an empty required email field.)
How to fix it
Change the hidden required field to optional, or delete the field entirely.
Or, set the required field's box (layer) to always be visible at every breakpoint.