I'm making a guess but I think you have two things that are preventing light hitting the ground. 1. The sun is just a little too low. I just did some experiments and if the sun gets too close to the horizon then suddenly it doesn't cast light anymore. 2. You may need to turn off shadows on the cloud layers.

I just had a quick play in the Realistic Sky dialog and came up with thie attached pic which I think is a bit like what you're looking for.

www.howiefarkes.com