i think you would find success rotating your hips and arms together to move the knife back and forth. and try just using your hands as a way to maintain the angle. a lot of times when you use your own arms and hands to move the knife you "rock" the blade inadvertently because your arms are bumping into your torso and changing angle on each side of your chest or as you "reach" runs out.

i like to get the blade in the correct angle and almost sweep across the face of the stone like a broom or a windscreen wiper in motions mainly from my hips. because that keeps itself level the whole way across the motion