If you are working with an Eloqua custom-object and you are having trouble interacting or getting data returned from them, it may be as simple as needing to re-authenticate.

When you POST /custom-objects in Eloqua, it does not have the new custom-objects immediately available. After posting the new object, you need to re-authenticate the instance to interact with that new object.  After you re-authenticate or create new instances, you can then interact with the custom object.