I feel you should make the shadows darker and make the light areas more lighter this would pop up the drawing
Good start! Overall it's good. Watch out for the reflected light in the shadows though. It's not as bright as you think. Squint your eyes and you'll see that the shadows are closer in value than it appears. Hope that helps!
Hey Martin, I like your Bruce Lee portrait, your reference is perfect for this kind of drawing. The problem your having is that you are not comparing your values to each other. Some areas are too light compared to others in the dark areas. I did a quick process sketch to show you how and when to add each value, I hope it helps :)