You will notice that if you register a plugin on the Create or Update event of an appointment in the Parent Pipeline, the plugins do not fire. When you create an appointment in CRM it is set to the status of “Scheduled” implying an appointment has been scheduled.
To trap create and update of Appointment through CRM you need to trap the Book Request and Reschedule request respectively.
When you run the diagnostic tool you would find that the Book Request is fired. It will show up the following xml.