Please check as follows if you have set up an overtime wage capping that causes your employee's overtime wage to be fixed:
Go to the Payroll module.
Click on Setup and select OT Table Setup.
Select the OT Table ID.
Check if there is a Maximum Total Wage specified.
Set the value to 0 or adjust the Maximum Total Wage and click Save.
Go to the Pay Record and recalculate the affected employee.