It seems that you are trying to show the RadBusyIndicator control while the UI is rendering which is causing this behavior. The BusyIndicator as all UI controls is rendered in the UI thread and if that thread is frozen it will not show.
I would recommend you to set the IsBusy not in the Loaded event of the Window as this event would be called after all of the controls are rendered (during which time the UI thread is frozen) but for example while the data from the controls is being fetched (possibly with the use of BackgroundWorker
). Hope this is helpful.