Is the employee set up as a Director? Directors can earn up to the annual earnings threshold before NI is deducted.
Is the employee set up as working abroad? If yes then NI will not be deducted.
To check these two settings go to Employee in the Tasking Zone menu bar, then select View /Edit employee details. Click on the View/Edit link for the employee concerned, then select the View/Edit button for the Employment Details option.