A comment in the History tab indicates why the attachments could not be sent.

Ariba Supplier Network (Ariba SN) imposes a file size limit (including comments and attachments) on purchase orders. If an order is routed through Ariba SN and the order exceeds the file size limit, the order is resent without attachments. Ariba Buyer emails a notification message to the requester of the requisition from which the order was generated, and to users who have the OrderFailureImmediateEmail permission (typically purchase agents), explaining why the attachments were not sent with the order.

If you want to include the attachments, you can send them by email or another method (outside of Ariba Buyer); or you can withdraw the original request, delete the attachment and instead include a zipped or smaller version, and resubmit the request.