Have you created a mapping for this view-mediator pair?

Is your mapping against the actual concrete type of this view? The mediatorMap doesn’t look at the superclasses or interfaces of the views hitting the stage, it only looks at the actual class

Is your view being added to the stage? The mediator onRegister() method runs after the view has landed on stage. It’s possible to addChild(view) without the view actually being added to the stage, if the parent is not on stage

If your view is definitely mapped to the mediator and is definitely being added to the stage: has your view hit the stage before the mapping was made?