I know this is late advice

since you're finished with this but for myself, when the lighting seems flat
the first thing that comes to mind is that the sun is probably obscured behind clouds.To check the Sun for sure, exactly and not just randomly move the sun about, you can use the preview window.
To make a sun check : (this is just my way - it works, but others might have better)
In the Preview window use Set Object Display Mode ( the small blue cube) to set the mode to show objects as bounding boxes. This causes the sun to show in the sky as cross hairs.
In the Sunlight node turn off Glow in Atmosphere if you happen to have it on. This is temporarily so the sun glow effect does not wash out the cross hairs in the preview window. Remember toTurn the glow back on when you render, if you are using it.
Use the Preview rotate left or right buttons as needed to turn the Preview directly toward the sun. You can follow your progress on the small compass. When the little compass sun is dead ahead, Use Tilt up if the sun is high until the cross hairs come into view.
Now that the sun is in the window let the preview finish rendering the sky in that position and then pause the Preview Window.
If clouds are rendering in the sun's position, you can click on the sun cross hairs and move the sun to a clear blue sky area directly from the Preview Window or you can alternately use the sliders or type exact numeric values in the Sunlight node. I think it is fastest-easiest to just drag and drop the sun in a clear blue area. Or if you want clouds filtering the light or special cloud shadows you can move the sun to a cloud edge boundary or little hole in the clouds or whatever on purpose.
Now you're done with the Sun Check you would return the Preview back to your untouched Render Camera view.
If you happen to have really thick clouds everywhere there might not be any clear blue area nearby, but that's a different problem that can be solved by localizing the clouds or various schemes for masking the clouds in the area of the sun.
Here is an example of a Sun position preview. Looks like my sun is just barely in a small clear spot.