+1  A: 

Whenever you see ${variableName} in the request, it means the variable hasn't been defined. Most likely, it's an issue with how XPath is setup. I've never used that component, so unfortunately I can't help you there.

However, I've found the post processor Regular Expression Extractor to be very easy and accurate.

The structure would like this:

1st Soap Request
  -- regular expression stored to "var"
 2nd Soap Request, using ${var}
BlackGaff