Looks to me like the fingers are not closed and the light is shining through the gap between the tips of the fingers and the wrist, showing in the thumb gap. Next, the lack of arm shadow on the wall is one of perspective. Draw a line from the light to the arm to the wall. Draw that in your mind (3d). If your wall is too far away, or the light between the arm and the wall in terms of orientation, you won't see a shadow. Also, make sure 'cast shadows' is enabled. By default it is so if you didn't make changes in your light object then you should be fine. Really, from what I see here, you have a wall way back in the distance, a light in the middle of the work area and an arm above the light but slightly closer to the camera than the light. This, to me, is a matter of depth of your objects. Hope this helps.