OK, I think I see the issue here. This works if I include all the namespaces in the expected result, eg:
<IdValue name="BackgroundCheckOrderNumber" xmlns:ns4="http://purl.org/dc/elements/1.1/"
xmlns:ns3="http://www.w3.org/2000/09/xmldsig#"
xmlns:ns2="https://www.sterlingdirect.com/sif/ws/hrxml/Screening/1.0"
xmlns="http://ns.hr-xml.org/2006-02-28"
xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">11742100</IdValue>
even though the actual XML response has
<IdValue name="BackgroundCheckOrderNumber">11742100</IdValue>
I would have thought that the match value could be constructed using the namespace prefix like this
<ns1:IdValue name="BackgroundCheckOrderNumber">11742100</ns1:IdValue>
but that gives me the mismatch error.