Value! Light and dark! Everything with few exceptions, is affected by the light source shining on it and an artist must show that to the viewer with VALUE. Subtle variations from light to dark on the surface away from the light source. I refer to 10 as the darkest and 0 as the white of the paper. Practice a gradual change from dark to light (or light to dark) to master this technique and you will be comfortable using it more often.
I know this and a teach this (or harp on it) all the time, yet a day ago I posted a small watercolor and BOING! I had to head back to the studio to correct the whites and not so white areas. This is often referred to as a salvage technique.
I am going to post this because I also tell my students that we all should learn from our mistakes. I hope you can see the changes I made.
Try this on an older painting that you have around the "studio" and are not that pleased with and PHOTOGRAPH IT NOW. Mix up a thin, watery value of blue or violet (which ever cool color works with the painting) and glaze over the areas that should be "darker" and are AWAY from the light source. Brush stroke in the direction of the area you are glazing over (curve with the curves, etc)>
Now photograph the improved painting, send both images to me and I will post your work here.
Good luck and have fun!!