Custom fields not saved

Hi

I created 3 custom fields (text input) following this method : https://www.salonbookingsystem.com/salon-booking-3-19-introduces-custom-fields/

I can see all 3 fields in admin panel and add booking through wordpress admin. I only use admin booking system, so I’m adding bookings myself. Whenever I fill in the form and save it, everything saves apart from custom fields – they get blank when I come back to check the booking.