Try this right-hand technique, especially in the upper register: find the middle of the string, put your thumb on the string, but NOT the knuckle of your second finger. Swing your fist through as you turn your wrist, so you hit the middle of the string with your knuckle as you pluck the thumb. I am able to get clear harmonics this way on notes as high as A and B, but never with the usual technique. I describe it in more detail in my book Solos for Pedal or Lever Harp.