Why patients miss appointments
It is worth being precise about the causes, because each has a different fix. In most clinics the reasons cluster into forgetting, a conflict that arose after booking, difficulty getting there, uncertainty about what the visit involves, and feeling better. Only the last of those is about the patient decision; the rest are logistics.
This matters because the instinctive response to no-shows is a policy about patient behaviour, when the higher return is almost always in the design of the booking and reminder process. A clinic that reduces its booking-to-appointment gap and sends two well constructed reminders will usually see a larger improvement than one that introduces a charge.