Send Notification To Client Not Working

Hi, I have the Booking Ultra Pro software and have it setup on a client’s website. It is working well, in that when a test booking is made and paid for the customer gets a notification that a booking has been made, however the client admin is not getting a notification of a booking, also if I then go to the booking in Admin and reschedule it or add a note to be sent to the customer and tick the ‘Send Notification To Client’ chcekbox, the customer doesn’t receive an email notification.
I have tried this using the generic PHP mail() function in Notification settings, also by installing & configuring Postman SMTP and choosing ‘Send via third party plugin’ AND by trying to just configure notifications via SMTP using that option and filling in the smtp details on the notification settings page, but alas still no notification is received by admin or the customer.
Can you assist please?
Chris