Try the following steps:
- Confirm the tax calculation using the HMRC tax calculator: https://www.gov.uk/guidance/work-out-an-employees-income-tax
- Previous Pay and Tax: If your pay includes Previous pay and Previous tax from another job – whether from a P45, or sent by HMRC as a P6- then you must also add those figures to the current employment year-to-date YTD figures when using the HMRC calculator.
- Follow the steps at https://paiyroll.com/kb/my-deduction-nic-paye-is-wrong/ to find the root of the problem.