Thanks for the response @samliew
I meant the error message that appears when submitting the form is not successful. i.e. “Oops, something went wrong when submitting this form”.
So the form never gets sent and the responses are not recorded in the settings panel within the Webflow dashboard. Yes, the site is hosted with Webflow.
Solved! For anyone who was wondering, it was all to do with the reCaptcha. It seems that if you have a reCaptcha on one form on your site, you need to have it on all forms on your site for each form submission to be processed.