Try each of the following steps in order until the issue is resolved:
  1. Open the report parameters
  2. On the General tab, note the date entered in the Calculate aging as of: field. Invoices are included on the report if they have a balance at the time of the specified date.  Any invoice created after the aging date would show a balance. To limit the invoices that are included on the report, enter a date for Include transactions with these dates
  3. If the invoice balance was reduced to zero by a credit memo but is not marked as paid, pay the invoice by printing a check to blank paper.  This changes the status of the invoice to paid.
  4. If the credit memos which paid the invoice was purged, change the transaction date range of the report to exclude these invoices.
  5. Delete any duplicate invoices
  6. On the Payments/Credits tab, if voided payments exist, that Date must fall prior to the 'Calculate Aging as of' date on the report parameters. Open the voided payment and select the History of Changes tab to see when it was voided.