I replaceed the two 3-way switches in my hallway and now they don't work. At one end of the hall the light will turn on or off and at the other end it will only turn on (sometimes off) but not both. I thought both swithes should turn on and off independently. I have attached three wires on both switches -- the hot wire to the dark colored screw and the other wires to the remaining two screws. Why wotn this work?