Thanks for the feedback. I will try to improve with your tips.

Looking back at this drawing i completely agree with you. It's true that I used a low opacity black layer for the shadow. I think you are right and I should draw them by hand like the rest of them. Also I should be consistent with the light direction. 

Thanks for taking the time to comment. I will try to apply your advice in my next drawings. I'm really grateful for your feedback.