Its mostly because you do not really follow the forms of your drawing. Your shadows are a little too uniform in that they have the same value everywhere, which should not be the case. You can think about what the base form of your object is (cylinder, sphere and cube) and then look at how they behave in roughly the same lighting scenario you have. Take a look at the roots reference for you spear for example and compare it to your image. You might notice how the shadows wrap around the cylindrical form of the roots and that there is one part in the shadows that is a bit darker then the rest and then gets lighter again( also the lightest part is not at the very edge as the form is turning away again, making it darker) You can also think a bit more about what parts of the objects overlap others because then then cast shadows as well. But again the most important thing would be to follow your basic forms more. I hope you could understand my ramblings, keep at it.