The human eyes' see in 3D, a perspective that can be difficult to translate to 2D because of the lighting conditions, and because we tend to move around a bit. That often changes the perspective of the model and makes it difficult to follow the geometry. Practice is the only way to get better. Keep it up and eventually you will master it.When I did the life drawing session I'd often go really well with a drawing and then all of a sudden things would go wrong! Proportion would go or I simply could not translate the shape / outline onto paper.