When you receive a payment with several invoices paid but the customer noticeably skipped one, call them that day. Ask them if they are disputing your price, your quantity, your quality or ask if they are just missing the one invoice that was not included in the payment. Contact their AP person that day, thank them for their payment and ask politely why they missed your invoice. If the reason is for anything else than a missing invoice, inform the right people (i.e. price, quantity, quality, wrong shipment, failed product, wrong order, etc.)