So, those MSI motherboards are bitchy little units. I have 12 rigs using MSI Z270-A and the initial setup is a bit of fiddly crap to make work.

What we do here is.

• Boot with USB cables removed from cards. Risers can be plugged in.
• Upgrade board firmware
• Install OS
• Install drivers for onboard video
• Go back to bios, enable over 3g video setting
• Enable integrated graphics as primary video
• Change PCI risers to GEN3
• Shut down machine.
• Connect 1 video card
• Boot to windows and verify it shows up
• Shut down and reconnect all risers.
• Power back up.
• Enter bios and verify board explorer shows all PCI slots full
• Boot to windows
• Install drivers
• Profit

I hope that helps. They sometimes work in GEN1, but GEN3 seems to give the most consistent results.