It is important that obj->is_of_type(CollisionNode::get_class_type()) returns true, which ought to be the case if you have called init_type(). But you can verify this.
Other than that, I don’t know what’s wrong. Maybe you can get a clue if you set “notify-level-collide spam” in your Config.prc file, and compare the output from the CollisionNode vs. the MyObject cases.
David