Appointments+ form does not appear in certain browsers

I've created an Appointments+ form on http://www.maharishiayurveda.co.uk/consultations/dr-donn-brennans-consultations and it works fine.

Trouble is, the form only appears in Chrome and only on my desktop Mac. It does not appears on Firefox or Safari. On my laptop Mac it does not appear on Chrome, Firefox or Safari. On a friend’s PC it does not appear on Chrome, Firefox or Explorer.

I tried deactivating all plugins apart from Appointments+ but this made no difference. I tried changing the theme from Avada to Twenty Fourteen and Twenty Twelve but this made no difference.