Introduction to XML in Flash
       by senocular

XML Node Reference Not Returning Value
Though its possible the reference is not correct (something you should tripple check) this problem is usually associated with XML loaded into an XML instance whose ignoreWhite property was not set to true. When ignoreWhite is... ignored, its default is used with is false. This means all white space between adjacent elements are treated as text nodes throwing off your nodes count and disrupting the order of where you think everything should be. Set ignoreWhite to true and you should be golden.

my_xml.ignoreWhite = true;