It is doing the normal range of motion, just not contacting the entire portion of the glass.
The wiper is 15 inches, but at the top of the glass, in the middle of the wiper's travel, there is an inch loss where it isn't contacting. At the bottom of the glass, inwards towards the center point, it is fine.
Does the car ever go through an automatic car wash? If so, the arm may be bent or worse. Check it carefully. See if the nut is tight against the spindle and the car body. There may be a way to lubricate. As far as the blade, OE is best for rear windows.
I don't know the history of the car. The nut is tight.