Try to user risers as well for that card. If it still doesn't work, switch the risers between them or try new risers.
I'm using windows, 6 gpu's on the same board (ASROCK H81 PRO BTC) and it's working perfectly.
Good luck
Got it, it was the riser!!
What also threw me off, was that ETHOS was saying that card 1,3,4,5,6 were working, so I assumed that corresponded to the PCI slot... But it didn't! It was actually the PCIE-1 slot card that wasn't working...
Now have all 6 mining perfectly!
Thank you guys for the help!