You'll have to check the Leave Category settings if there's a leave loading rate set up for the specific leave category. You can also check the individual employee's leave allowances if the Can Apply for Leave is turned on.
Here's how to adjust leave loading rate for all employees:
Click on the Employees tab, and click on Payroll Settings.
Click on the Leave Categories.
Select the Leave Category that is not adding to the employee.
Locate Leave loading rate field, adjust the rate if needed.
Click on Save.
To adjust the leave loading rate for individual employee:
Click on the Employees tab.
Click on the employee name.
Click on the Leave Allowances.
Check under Leave Allowances Template if the employee is using a leave allowance template or not.
If not, find the leave allowance not adding and wish to adjust the Leave loading.
Click On Save.
Then, attempt to create a dummy pay run to verify if the leave loading is adding up.