If your wiper isn't clearing an area of the screen, even though the blade is passing over it, then the problem is the angle of attack, rather than the wiper head (the trapezoid doodah with the blade attached to it).
Try a new/different wiper arm. They should be easy to find.
If you mean there's a part of the screen that the wiper just doesn't reach, then that's down to the position of the arm on the motor splines. There may be a round mark on the windscreen that the wiper should line up with when it's parked. Otherwise, mark it yourself (marker pen), remove the arm, rotate it anticlockwise to the next spline and refit it. As long as it doesn't hit the windscreen rubbers on full swing (run the wiper on fastest speed with a very wet screen to get the proper effect) then it'll work.