java.lang.NoClassDefFoundError: com/googlecode/objectify/ObjectifyService

I am a new bee in the world of objectify (just entered today). But no doubt this is a cool stuff for app engine. I started with the Objectify version 3.0. I added the jar file using eclipse external JAR file adding mechanism. Right click on the project and see in the build path. On the first hand got this exception

java.lang.NoClassDefFoundError: com/googlecode/objectify/ObjectifyService

Cure is you have to copy the objectify jar to your project's WEB-INF/lib folder.
You will readily get rid of this error and encounter another one

java.lang.NoClassDefFoundError: com/google/appengine/api/datastore/Async DatastoreService

But here is the cure is not so good. I down graded the version to objectify 2.2.3. And its working now.
But really don't know why..