Why would a module in a JAR not be found?

Why would a module in a JAR not be found?

I am working in NetBeans 7 with Python script using Jython. I am trying to
build a NASA WorldWind example.
I have added JARs to the classpath, including the one that contains
gov.nasa.worldwind.
The code crashes, saying
Traceback (most recent call last):
File "C:\Users\wrightky\Documents\NetBeansProjects\WW\src\ww.py", line
4, in <module>
import gov.nasa.worldwind as wwj
ImportError: No module named gov
I have both added the JAR with "gov" to the classpath and even added the
JAR to the Python package manually, under which I can see
gov.nasa.worldwind.
I could add my code but not sure if it's relevant.
Why would this happen?