When an Invoice is rejected, SSI will provide details explaining the rejection in the comments section at the bottom of the invoice. If you have notifications setup, a notification will also be sent to you. If you are not clear on the reason, you can either enter questions into the comments section on the invoice which will be sent to the PO requestor or alternatively contact the requestor for further details to resolve the matter. Once confirmed, all you need to do is to create a new invoice to replace the one that was rejected.