this indeed has to do with students being marked as "attended". I suppose your scheduler is set up to allow only one booking (or a fixed number of bookings) at a time. If a student has made a booking, they will no longer be listed as needing an invite / reminder since they can no longer make a booking. Only once they have been marked as attended, they can actually make another booking (and can be invited/reminded).
One option would be that you mark past slots as attended, or delete them if they didn't take place.
Another option would be to allow unlimited appointments to students.
I hope this makes sense.