If the distance between the (soft) spotlight and the target object is less than 1 blender unit (just a guess), the light is not bright enough.
This is a big issue in my projects because 1 blender unit = 1 meter. Which means everything under 1 meter is to dark.
On the "rendering.jpg" you can see that closest plane is the darkest which makes no sense.
I tested with this yafaray 0.1.5