one assumes they haven't done this from day one, although you don't say how old they are, so they need replacing. However, as the 'problem' seems to be on the return stroke, then it may well be that the arm needs tweaking. Stop the arms at the vertical and remove the blades. Gently set the arm on to the screen and see if it sits very squarely onto the glass. If it doesn't ask the dealer to reset them, or do them yourself. IMHO replace blades with Bosch blades and change your screen wash liquid.