When an invoice is partially paid, the 'Maximum Payment Amount' in the control payment is set to 0.00. This is because there usually a reason why the invoice was not fully paid, so some intentional action is required to ensure that the remainder is not paid by accident.

Open Control Payments and find that vendor invoice. Change the Maximum payment to the outstanding amount. You are good to go.

If you are using the current version of Adagio Payables 8.1C, you can do the same control payment change directly within the Vendor Transaction function (ie Edit the vendor, go to the Transactions tab, see the Applied information for the invoice, and see the Control Payment information to edit as desired).