It may take up to 20 mins + for changes to be reflected on your online bookings pages.
Similar to making changes to Services, Staff Qualifications etc. Changes made to Special Offers can also take up to 20 mins to be reflected online.
Make sure you click 'Save' in Phorest and then refresh your online booking page or website to see if your changes are reflected.
A common mistake is to continually edit it, archive it. Try not to do this as the 20+ mins is extended every time.