Are you using Windows 10? If so, what version of the OS are you using, and also what version of Nvidia CUDA are you using?
I'm using windows 10 but I haven't been able to get back into windows since my original install. I've tried every combination of video card to pci-e slot when trouble shooting. I tried 1 card at a time in each pci-e slot, all 4 cards. Then I tried all 4 cards plugging directly into the mobo in each of the 16x slots. Then I tried a known working card in each of the 16x slots.
I then tried EVERYTHING again with a different stick of ram. Same results. No display, monitor doesn't get a signal.
Have you tried a cmos reset? its a pain to reinstall windows again but ive found its usually quicker to start again from scratch than troubleshoot problems on a new build. Also if you need to change bios settings i would do it before adding multiple gpus. That seemed to be my problem, but different mobo and gpus