Hi,
Thanks for your quick response. I found the error, I think. Mediator A always receives the notification before Mediator B. In Mediator A, a runtime error was generated because it tried to access a property of an uninitialized object. By commenting out that part, both Mediators receive the notification.
So I guess that Mediator B did not receive the notification because of a runtime error in Mediator A.
I also tried debugging to see the observer lists in the view, but I don't know where to find these lists.