I'm running a RX 580 rig with six video cards on a Z270-a motherboard. Two  of the cards stopped working, so I checked each one out separately in a PC I have, and they operate and mine just fine. I also tried other power cables from my PSU that are working correctly with other cards, but the fans would not spin.

 I also changed the risers for both cards. That is all I can think  of that could be causing problems. Any ideas why they are  not working? My only thought is bad risers, which I will change, but wanted to get opinions if I am missing anything.