Verified answer

Well, for starters, it shouldn't move "all the way" up to the steering box. A certain amount of space is normal. But assuming you mean back up to where the old one was, did you line up the splines properly? There's a "blind spline" on both the arm and the shaft that need to be lined up for it to go on very far at all. Other than that, the torque spec for Pitman arms are rather high, and the arm usually won't go all the way up until you torque the nut.