One thing you can try is to ensure that when the blades are in the vertical position on the screen, the arm holds the blade exactly perpendicular to the screen.
Usually when blades wipe fine in one direction but not the other, it's because the blade isn't flipping over correctly on the change of stroke.
If you set the blades in the vertical position (I turn off the ignition in mid stroke) you can then remove the wiper arms and use a pair of pliers or something to grip the wiper arm to twist it slightly one way or the other.
Mind you, I still find that in anything other than heavy rain they are a bit pants. I try to rely on Rain-X and not using my wipers for as long as I can.
