Have you attempted to connect the GPUs one at a time to make sure they individually are functional? Are you able to place the 2 GPUs that are not functioning properly into another system to see if it is able to get them working? What OS are you running, and does the OS detect the GPUs at all?

It sounds like it would be the cards, maybe they died during a bad flash recently or got a good shock while you were placing them in without discharging your static electricity. If they are unable to work in a slot that other cards are working in, then it probably isn't the slot or the motherboard. The PSU should have enough juice.