Basically what the subject line says - my motherboard isn't detecting any GPUs as PCIe devices at all. Help would be greatly appreciated 
Parts:
Mobo: MSI Z170A SLI PLUS
GPUs: RX 470, R9 Fury
For context, I originally started out with a RX 470, but I recently upgraded to a 4K display, so I decided to buy a R9 Fury instead. After using DDU to sweep the old GPU drivers from my computer, I tried plugging the R9 Fury in (with both 8-pin connectors) - and got no response; MSI board explorer showed nothing in that port. I then retried my RX 470, and got the same problem. Does anyone have suggestions?

Parts:
Mobo: MSI Z170A SLI PLUS
GPUs: RX 470, R9 Fury
For context, I originally started out with a RX 470, but I recently upgraded to a 4K display, so I decided to buy a R9 Fury instead. After using DDU to sweep the old GPU drivers from my computer, I tried plugging the R9 Fury in (with both 8-pin connectors) - and got no response; MSI board explorer showed nothing in that port. I then retried my RX 470, and got the same problem. Does anyone have suggestions?