I think the problem with the side views is the head shape. The ear is too far back, and the character's neck seems to bulge out above their torso. They also seem a bit wall-eyed, too much of their eyes is visible. The hair also seems to end halfway down the head, whereas in the back view it goes down to the neck.
The back view just looks a bit flat because the hair line is horizontal and because there isn't any shading. Try shading the lower third of the head to show its roundness, and perhaps have the head cast a bit of a shadow onto the upper torso.