User has set up interest to be charged on invoices if the payment is delayed via Terms & Currency in Projects > General > Billing Options, but when the payments are actually delayed, the invoices do not show the interest to be paid, nor is the interest amount added to the amount due.
This is because Core handles the interest to be charged on delayed payments (overdue invoices) separately using late fee invoices. You need to create the late fee invoices and send them to the clients in order to charge interest on delayed payments. Check the Core Help for details. You may also want to check out these articles: