Appointments+ Plugin - removed appointments

I've had a significant number of appointments automatically removed and I don't fully understand why this happens for a particular client.

Appointments are paid and set to auto confirm, so I assume it's an issue with the clients Paypal transaction?

The real problem is that clients don't seem to know it's happened - they assume the appointment is booked for some reason and only when no confirmation email arrives do they get in touch (sometimes weeks later), often to be disappointed that their desired slot is no longer available.

I would like to understand what is going on here so I can better respond to customer queries / complaints.

Many thanks,
Stuart