Hermann you’re not doing anything “wrong” here. You’re actually on the right track. The reason it feels flat is not because you need to go darker everywhere but because the shading doesn’t have enough difference between the darkest and lightest areas. Right now most of the tones are sitting in the same range so everything blends together.
Try this little tweak. Pick only a few key shadow areas and push them one or two steps darker. For example under the cheekbone, under the nose, under the eyebrows and the shadow under the jaw. When only those areas go darker the face suddenly gets depth and the smooth transitions become much easier because the drawing now has room to breathe.
And don’t worry about transitions. If you build the darker parts slowly in layers with light pressure your blending will stay smooth. You’re very close already. Keep going, this is a solid foundation. 😊