Cannot remove text that I deleted from within a shortcode

We have Appointment Plus shortcodes on and have 60-minute and 30-minute appointments. Here is a problem we have encountered:
Say someone books a 30-minute appointment at 09:00 and someone else books a 30-minute appointment at 10:30. This means that when someone else comes who wants a 60-minute appointment between 09:30 and 10:30, that time slot will not appear available, even though it is available. How can I overcome this?