xml xpath confusing

[guocnc]

I create an object of Microsoft.xmlDOM. load a xml file(validation no error).
and objXMLDoc.setProperty "SelectionLanguage", "XPath"

Then use
Set NodeList = objXMLDoc.getElementsByTagName("appointment")
to successfully retrieve a list of nodes with tagName "appointment"

However, when I change the following code
Set NodeList = objXMLDoc.selectNodes("//appointment")
to hope to retrieve the same information, but get nothing. I get the length of the nodes is 0.

If I change other xml files, both above selectNodes and getElementsByTagName will get the same results.

Anybody met the same problem before, what is the casue