usually you do one of two things depending on the sun position. 1) If the sun is low, you'll want a relatively high atmospheric haze setting...I think I used the color black for it too. You also need 'clouds cast shadows' on and keep the 'shadow lightness' at a low percentage. 2) If the sun is high, you'll need a high cloud base...say at least 30,000meters high for the clouds. Again, I would recommend a dark atmo haze color and set the percentage to around 50 and play with it from there. Oh, also have the sun strength at least 1000% for both of these cases. Play with that for a bit and let me know if you're still having issues. Hope that helps.