We eventually discovered this:
"This issue is happening because you have a 24/7 schedule with a 0.5 appointment window, which creates a lot of available appointment slots.
However, there is an appointment booking property [sys_properties] called 'sn_apptmnt_booking.max_appointments_returned' which limits the number of appointment slots available. The property has a default value of 100. This explains why the location queue is limiting the available slots in the schedule.
The workaround is to simply increase the value of sys_property 'sn_apptmnt_booking.max_appointments_returned'.
Something like 1000 should do the trick.