The sun is treated as a normal lightsource. You can check which light is which type (directional, point, spot), but you can't decide which of the lights are the first in the array. And this leads to your problem: Because the sun is most far away, it is not used, when there are enough other lights in the range of the model.
I already told jcl about it, but he said, it would be okay like it is now.
Sry, I didn't found a solution yet =/