Delivery note/service entry sheet does not exist

Hi Experts,

In ECC, a confirmation key is created which does not require a ASN.

PO is created and PO response is sent from SUS.

GR is created against the PO and a delivery note number is put in. Material document replicated over to SUS as Goods Receipt Notification.

A Invoice is created against the Goods Receipt Notification, which fails in ECC IDoc posting with the error "Delivery note/service entry sheet <material document number? does not exist".

Any idea why the system cannot find the material document when it really exists?.

We are in SRM702 & ECC 606.

Thanks