Function require assigned availability is used to find just those applicants with an available diary entry for the dates and time slot specified.
Can you ensure that applicants diary has available events entered for the days that they are unavailable:
Click Applicants.
Select the Applicant you want to search availability for.
Click the Scheduling tab.
Either:
Select the event on the available date and check the event type is available.
Double click the available date and create an Available Event.