Some controls handle the MouseRightButtonDown event and doesn't let it propagate (e.g. MenuItem). You can test this easily if you're calling a code behind method instead of using a trigger and see if you'll reach a breackpoint in that method.
In your xaml:
MouseRightButtonDown="UIElement_OnMouseRightButtonDown"
In your code behind:
private void UIElement_OnMouseRightButtonDown(object sender, MouseButtonEventArgs e)
{ }