Dear Marc,
Its clear in the data source definition that GL account and other account assignment are picked up from RBCO table. There for the POs which has no account assignment attached to it will bring blank accounts. Please refer to the below link
http://help.sap.com/saphelp_nw70/helpdata/en/57/dad06449114814919000e27dfb255e/content.htm
You have to enhance the data source to bring in the accounts details if the invoice its not in RBCO
Regards
Gajesh