When you receive a payment and the customer did not pay your invoices in chronological order, call them immediately. Ask them if they dispute your price, your quantity, your quality or ask if they missing one that you noticed not included in the payment. Customer AP personnel pay invoices that are approved. You need to know why a buyer told them not to pay you for an invoice. Approach them cautiously since there may be an issue that needs to be resolved between your firm and the customer. Contact their AP person that day, thank them for their payment and ask politely why they missed your invoice.