Resource not found for the segment

Hi
In Gateway, I want to get header and item data of my PO from ERP, on behalf of the PO number.

In transaction /IWFND/GW_CLIENT I execute the http method GET for

/sap/opu/odata/sap/ZGW_BD_PURCH_SRV/

and
/sap/opu/odata/sap/ZGW_BD_PURCH_SRV/$metadata

I got a http status 200 return – which is fine 

In the ERP that I am calling, there is a purchase order with no. '3000000006'

But when I execute the service for following url: /sap/opu/odata/sap/ZGW_BD_PURCH_SRV/PurchaseOrderCollection('3000000006')

But I got this error:

<?xml version="1.0" encoding="utf-8" ?>

<error xmlns="http://schemas.microsoft.com/ado/2007/08/dataservices/metadata">

<code>005056A509B11ED199D891D139C92103</code>

<message xml:lang="en">Resource not found for the segment 'PurchaseOrderCollection('3000000006')'.</message>

</error>

My settings are like this:
Entity types is created:
PurchaseOrders (PoNumber is key) + PurchaseOrdersItems (PoNumber+PoItem are keys)

Entity Sets is created:
PurchaseOrderCollection (assigned to PurchaseOrders) + Purchaseorderitem (assigned to PurchaseOrdersItems)

In the Service Implementation the PurchaseOrderCollection has following mappings:

And for Purchaseorderitem I have following mappings:

The only BAPI that we are using is: BAPI_PO_GETITEMS

Any ideas?

Thanks,