I've always found pulling the hinged part of the arm up off the screen, pushing the bottom part of the arm towards the scuttle, then giving it a tap with a nylon hammer will shift all but the most stubborn wiper arms. Some masking tape in over the area in the firing line of the hammer for good measure. Oh, and don't forget that you can't use the wipers to put them in the service position if the bonnet is open.