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.
Yeah one of the first thing I tried is plugging only those not-detected GPUs one at a time but both are not getting recognized by Windows 10 Pro (64 bits). I've tried to put those GPUs into my other rig but my other rig doesn't detect them also. I am running Windows 10 Pro (64 bits), the OS only detects 3 GPUs out of 5.
When I was placing those GPUs on the motherboard the power supply was always turned off to avoid a shock. Something that makes me ask myself some questions is that those 2 GPUs when I am booting, their fans are running OK and all lights are on the GPUs and the LED light on the PCI-e risers are on also....
Thanks