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) right next the Tax Amount in the calculation payslip:

A typical example why tax is not being deducted would be because the employee was over taxed in his previous employment or previous payrolls, hence the system will compensate the tax. This also happens when the employee switches a tax bracket mid year.