Nope, the message type shouldn’t matter.
XPath inside an Orchestration is hard to do sometimes due to the namespaces inside the messages.
You might want to look at your schema, find the node you want to xpath, and then look at the system generated xpath to that node.
Just click on the node inside the schema editor and you should see the xpath under properties someplace. It will be long and ugly – including all the namespaces.