It's been years I've had to wait to get an appointment with my doctor, but you need to know how scheduling is done.

The receptionist does all appointments the day before or earlier, but she can't book all the slots. Several slots can only be filled by the doctor's nurse and are only filled on a same-day basis. I call the receptionist and if they happen to be booked solid that day then I can call the nurse. The nurse has commented before she often fills her slots by noon, so if I were to call after that it would need to be an emergency or such and they'd just squeeze me in (i.e., that's why the doctor is late).