When you build neethi Assertions, do you find instances of "org.apache.neethi.builders.xml.XmlPrimtiveAssertion" instead of proper Assertions? The issue is that your runtime has not picked up the "org.apache.neethi.builders.AssertionBuilder" file inside the META-INF/services directory. This problem mostly occur in old versions of Eclipse or in OSGI environment. So check if you have added the META-INF directory that contains the "org.apache.neethi.builders.AssertionBuilder" file to your classpath.
If you want to know the technical details, then please refer how Apache Neethi implemented the http://java.sun.com/j2se/1.3/docs/guide/jar/jar.html#Service at org.apache.neethi.util.Service.java .