Is the invoice that has not been paid yet displayed on the payments screen? Would you like to pay that amount?

Service fees for the previous month that have not been settled will be charged in addition to the next month's bill by default. If you still have an invoice marked unpaid on the Billing & Payments screen, you can pay for it by paying the entire bill for this month.

On the Billing & Payments screen, click the detail button on the unpaid invoice you want to check

Before moving to the detail page for this month's bill, there will be a dialog shown for asking you that you willing to pay for the overdue charges.