There could be two reasons for why the values on your IR8A form seem to be incorrect.
a. Either: You have not processed payroll for the full year. In this case, you need to
i. Manually calculate the value of all pay elements for each employee for the missing months.
ii. Proceed to Payroll Setup -> IRAS Opening Balance -> Fill up opening balance for missing months.
b. Or: The tax code for some pay elements have not been tied up correctly. This means that when setting up your pay element, you have not given it a proper tax code. To fix this,
i. Generate report 18 and 14 for comparison. If certain pay elements in Report 14 are missing in Report 18, these pay elements have not been tied correctly to a proper tax code.
ii. In this case, you need to manually calculate the value of each untied pay element for each employee.
iii. Proceed to Payroll Setup -> IRAS Opening Balance -> Fill up opening balance for missing months.