Xpath doesn't return anything in Nokogiri
If you don't want to deal with namespaces, you can use
doc.remove_namespaces!
It is Lazy (= efficient), but it is not recommended.
Your XML is in the namespace some_namespace
but your XPaths don't have a namespace binding. You're essentially querying different elements than are in your XML.
Using "Clark notation", the element you're trying to get to is
{some_namespace}ShortMessage
but you're querying for ShortMessage
in the no-namespace.
Just because there is no prefix, i.e. the namespace is the default namespace, doesn't mean you can ignore it.