Does the day fall on a day they do not work?

Check the employee's working pattern in their profile via the More tab. Breathe may not think they are working on that day of the week.

Do they receive Bank/Statutory holidays? Check whether their booked leave coincides with a statutory holiday date.

Is there is a company holiday booked?

Click here to take a look : Company Holidays

Does the employee have 'ignore working pattern' ticked? If so, they are required to manually input the time missed in their original Leave request. You can check this by viewing their leave request in their profile.