WPF Problems using IRegionManager

I use this method to call a view:

IRegionManager regionManager = ServiceLocator.Current.GetInstance<IRegionManager>();

//Esta es la interfaz que implementara el CodeBehind de la vista.

IListaBeneficiosPopupView busquedaBeneficioPopup = ServiceLocator.Current.GetInstance<IListaBeneficiosPopupView>();

regionManager.Regions[RegionNames.FlyoutRegion].Add(busquedaBeneficioPopup, "busquedaBeneficioPopup"); 

regionManager.Regions[RegionNames.FlyoutRegion].Activate(busquedaBeneficioPopup);

My goal is that the views load in the following way and in the following order:

  

View A - > 2. View B -> 3. View C

  

but the current result is that when I load the view C , it opens behind the view B , when it should be displayed above.

What could be happening?