Appointment's username doesn't automatically fill in

In Appointments+ once a user has completed an appointment, if I go into the dashboard under, "Appointments" --> "Completed Appointments" and click "See Details & Edit", under the appointment information section for the appointment's Client Information "User" it says "Not registered user." All of my appointments require them to be logged in, so how can I get this to be automatically set to the user's actual username? This may be for the active & removed appointments also, but currently I only have a completed appointment.