Ok, you got me. First of all what is and 8 1/2 N? Next are you saying that the disk only penetrated in the ground 2 inches. If so what type of soil do you have, rocky, clay, gravel? Is the ground too dry? Do you have the disks in a straight line or angled? When angled they dig in a little more. Is the ground plowed first or fresh nothing done to it first?
Ok another thing to point out is some 3pt disks just dont work as well as pull behind. I have noticed 3pt ones tend to not penetrate as much and don't level and smooth out the ground as good as a pull behind.
So you need to answer a few more questions and possibly take some pictures to explain your problem.
Hope this helps.