are the conical washers on in the right order? They should be in this order (though you can swap the thick and thin ones depending on how wide the rim is).
If they are in the correct positions, then sometimes it takes trial and error, you may have to over compensate when setting up the pads so that when you tighten them, they move into the correct position.
I always set up my pads one arm at a time just by holding the arm and pad against the rim to initially set the position and tightness (without the spring attached, just makes life easier), then connected the brake and pulling hard tightened the pads (this stopped them from rotating so I could get the pad nice and tight).