As in it skips in a few small areas, or it's staying about 4" away from the glass?
If it just barely misses the glass, most likely the pivot point is corroded and the spring pressure isn't strong enough to pull the blade onto the glass. For that you must replace the entire arm. If you can lift it up and it springs back freely, look for the same corrosion and binding on all the small parts that hold the rubber insert.
If the arm moves freely and springs back but stays around 3" or 4" off the glass, look for a little slide, or tab, right next to the pivot. Its purpose is to hold the arm up so you can wiggle it to pull it off. Raise the arm a little more, then slide that tab sideways about 1/4". That will let the arm come back down.