A project being billed has a PO number associated with it, but it doesn't show up on the invoice. Why?
It depends on whether the invoice was created before or after the PO number was added to the project. If it was created before the PO number was added, it will not show up after the fact and would need to be deleted and reprocessed. If it was added after, it should show up, provided you are using the correct invoice template. Do note that only the PO number of the parent project shows up on phased invoices.