All PCI Express ports are working except the grey one - Asus Prime z270-A + GTX 1080TI

Hello, i've been using my mining rig without problem with 4 GPU, yesterday arrived my other PSU and i connected 2 more GPUS to my motherboard ASUS Prime z270-A.. I've realized that the GREY port 16_1 doesn't work, but the others port work!! So is nothing about the PSU or the GPU... But the weird thing is that the grey port WORKS only if i connect my GPU without PCI-E riser and other gpus are disconnected..

Anyone can guide me to make that port work with a pci-e Riser?

if on windows

plug all the GPU (with the risers) > then go in the devices manager and look for something called (VGA) or something with a "?" > right click and update the pilot