I have wired a 3 way switch in the basement. Using 12-2 from panel to switch. 12-3 from switch to other switch on other side of room.12-2 to the lights. One switch only works both when in up position. If I put the one swwitch in the down position the other switch doesn't work at all. I have the 12-2 hot wire into the Black terminal and the red and black 12-3 wires as the travelers going out the top two terminals, going to the other switch. At other switch I have the red and black going into the top terminals and 12-2 black connected to black terminal of the switch. Again the lights come on only when the switch closest to the stairs is in the up position. Can anyone tell me what I've done wrong. I've watched 9-10 different you tube videos and I've wired the switches the way they showed and still problem.

 

Your wiring sounds correct. Did you connect the white of power in to the white of the three conductor and the white of the three conductor to the neutral to the light at the other end? It really sounds like you accidentally connected the wrong wire to one of the common terminals, Since you have two blacks in each switch box go back and double check you didn't accidentally swap them in one of the boxes.