There are a number of reasons why this could occur. Please investigate the following most common causes:-
1) You have missed out/skipped/not updated a pay period. If your employee is on a cumulative tax code, a skipped pay period may result in no tax being deducted, or even a tax refund, the next time their pay is processed.
2) There has been a change to your employee's tax code. If so, the employee's tax free pay allowance may have increased leading to no tax being due this week/month.
3) The employee is not earning enough to pay tax. If the employee's earnings are below the tax free pay allowance, they will not pay tax. The employee's tax free pay allowance is determined by their tax code.
4) You have just started using our software part way through the tax year. If so, please check to make sure that you have entered the employee's year to date values correctly.