When creating a new Service File via the Add Service Wizard link, when the first Event is booked it is not being booked for the Site selected in the Availability Site box and is instead defaulting to the Worker's assigned Site.
If the Worker who is booking the Event...
has "Restrict Worker by Site" enabled in their Security Class
has a Site listed on their Worker Profile that is different than the Site being selected in the Availability Site box
Then the correct Availability Site will need to be selected again in the Event Site dropdown on the Event Details tab before proceeding with setting up the Service File and booking the Event.