Import Standard Purchase Orders Issue

hi all,
i've been trying to figure out how to properly import records to po base tables (po_headers_all, po_lines_all, po_distributions_all) but with no luck.

i have a custom package that gets data from custom table and inserts them to the po interface tables (po_headers_interface, po_lines_interface, po_distributions_interface). after inserting the records to the interface tables, i run the concurrent request "Import Standard Purchase Orders" supposedly to import them to base tables. however, i am facing the issue wherein the records in the po_headers_interface and po_lines_interface are the ones only being imported to their respective base tables. the po_distributions_interface isn't.