Turning the head is not the same as lifting the head. Lifting the head is jerking your head up to look toward the other side of the net early. It brings it away from the torso, makes it difficult, if not impossible, to see the ball, and forces the torso and the arm to lift out of the swingpath. The head can move with the torso, but it should not lift up and turn away (facing the side fence, in bad cases) before striking the ball. This is likely what the OP is doing. If he was just turning his head tracking the ball, it wouldn't be causing him to be unable to hit the ball in the sweetspot now, would it? The pros keep their head very level, not lifting up prior to contact.
Often, this can come from having a contact point too late and close to the body, where the head and body must move out of the way to contact the ball.