What I would do is...

Check that shadows (and soft shadows) is turned on in the sky lab. Turn off shadows for all the lights. Then turn shadows (yes, make them soft) for one or two of the lights off to the side.

It'll render alot faster and that many soft shadows being cast are probably washing each other out.

Is the picture linked to, a bryce render? If it is, make sure that the ground is accepting shadows. Or you might have shadows covering the entire ground.