There are multiple reasons why an invoice or credit may not have exported to your accounts package please check each of the following:
Check the Invoice Date to ensure it has been submitted correctly. Note also the default date range of the export is 365 days past the current date.
Check the Export ID reference at the bottom of the details tab of the order, has it been already exported or has it been excluded this would be displayed as -1.
Check the Status Log tab to see when the invoice moved to the status Completed, as only competed order and credits are included within the export.
If you are exporting by Purchaser Group, check that the site is active within the Purchaser Group.