Here are a few common reasons of why something might be showing up on your AP or AR Aging report when you don't expect to see it there:
1. Check the Apply tab to make sure that transaction(s) are fully applied to it and that the amount remaining (circled in yellow below) is 0.
2. It is a negative invoice or credit memo.
A negative invoice is a credit memo and needs to be entered that way, and a negative credit memo is an invoice. If these have been entered as negatives in the past, then they can be removed by deleting them and re-entering them as positive amounts of the opposite type.
3. Click on the Distribution tab of the transaction and make sure that there aren't entries on both the debit and credit side to Accounts Payable.
4. The Apply Date (circled in orange below) of the corresponding transactions is after the date you are running the Aging report for.
For example, a user might create an invoice with the date 5/14 and then create a corresponding payment with the date 5/20 (underlined in green below). If you apply that payment to the invoice, it will have an apply date of 5/20, meaning that the system does not recognize the payment being applied to the invoice until 5/20. That means that from 5/14 - 5/19 you have an unpaid invoice that will show on the aging if you run it as of one of those dates.
So with the Payment and Invoice (#900) example above you can see that if we run the AP Aging report As Of 5/15, the invoice will show up on the aging (because the apply date of 5/20 doesn't happen until after the As of date):
But if we run the same report As of 5/21 (after the apply date) the invoice will no longer show up:
If necessary, you can change the Apply Date by clicking on the Apply Date and typing a different date or selecting it on the calendar that opens up. Then save.