When adding new cards especially if you mix (nvida and amd) uninstall the drivers with
DDU, and then reinstall the drivers. I'd also add each card in turn so you can work out if you have a bad riser or card. Also if you can test with 1 PSU to rule that out
all cards are functionning because i was using them previously on my computer.
i bought a RX 570 to replace the two R9 280x and then wanted to add the two R9 280 to my mining computer
all riser are working also tested each one by one
only both card not working together (ok for the 580 1050ti detected but crash every 5 min, and R9 280x not detected at all)