Trouble updating quantites on Return Authorization

Within web services when trying to zero out quantities on Return Authorization line items I receive the following error message:

INSUFFICIENT_PERMISSION "You do not have permissions to set a value for element item.quantityreceived due to one of the following reasons: 1) The field is read-only; 2) An associated feature is disabled; 3) The field is available either when a record is created or updated, but not in both cases."

Here is the code (C#):
                        //Pull down the RA in order to work with the line items in question
                        RecordRef rec = new RecordRef();
                        rec.internalId = internalId;
                        rec.type = RecordType.returnAuthorization;
                        rec.typeSpecified = true;