Why is one of my employees missing from the Payment Summary report? 

If an employee's Tax Scale is set to 74 or higher, they will not receive a Payment Summary. To check what Tax Scale has been assigned to an employee, go to Payroll > Select Employee > Tax and Super > Tax Scale.

If you need to apply a specific flat rate which is not already available in the Tax Scales list, you may need to Set Up a New Flat Rate Tax Scale.