No success/error messages showing

I recently became aware that for some reason my Mailchimp for WP plugin had stopped populating my MC groups as it always did before, and that it had also stopped showing success/error messages when filling out the form.

I fixed the first one by using the new interest categories field on the form, but the success/error issue is still there. If you take a look at http://adamcroft.net/vip-club/ and fill it out, there’s no indication as to whether or not the signup has been successful, whereas beforehand there always was.

How can this be fixed please?