Why did my EDI invoice not get paid?

Almost always because it failed the three-way match. Your 810 invoice gets compared against the purchase order and the receiving record. If the price, quantity, item number, or PO number disagrees with either one, the invoice does not get rejected loudly. It goes into a research queue, and you discover it when accounts receivable asks where the money is.

The 997 acknowledgment that said 'accepted' did not mean paid. It meant your file arrived and parsed. Confusing those two is the most expensive misunderstanding in supplier EDI.

The durable fix: generate the invoice from the purchase order and the ship notice so the numbers cannot disagree, and monitor the whole chain, not just the receipt. That is what a managed connection does all day.