There can be several reasons why tax is not being deducted for a particular employee.
To confirm how the tax is being deducted, it is recommended to check the info button (i) to the right of the Tax amount in the Calculation Payslip:
A typical example for this would be that the employee was over-taxed in their previous employment or previous payrolls, so the system will need to adjust the employee's future tax payments accordingly.
This also happens when an employee switches a tax bracket mid-year.