If the hang backs are only in the first 6″ (150 mm) of the pile, my first conclusion might be that the material will not deliver with such a substantial drop off from the delivery section to the skid. Try to get whatever additional height can be gained by raising the side jog blades to the top of the slots. By making this adjustment, you may be able to gain an inch or so in height.
Another inch may also be gained by using wedge blocks or full length sticks down the outside edges of the pile. As the pile builds to normal delivery height, the sticks can then be easily pulled from the pile.
If the sheeter is equipped with a variable speed delivery, then a reduction in the amount of overlap during initial start-up of a skid may reduce hang backs.
While everyone’s objective is a neatly jogged pile, you should check the position of the side jogger blades to make sure they are not set too tightly. Open the jog box slightly so that the sheets being delivered do not get pinched to a stop before they reach the front stops.
