When a patient contacts the clinic telling you that he/she did not receive an appointment confirmation, you're able to resend this confirmation.
To resend an appointment confirmation, search and find the appointment you wish to resend in the calendar. Click the appointment and scroll down in the pop-up. Click on the send again link.
Note: while walking through this procedure it's advisable to check the existing email address. Also keep in mind that e-mails could end up in the spam box. Especially for certain email services like Gmail and hotmail spam filters sometimes do show unexpecting behaviour.