I have this problem too. Is not the card, I tested all of them individually and they worked, they just don`t work the 6 together. I use simple mining OS, so I think the problem is on the mobo bios. I saw a lot of youtube videos and have tried to change some setting on the mono bios but did not had success. I read somewhere that i should set "enable 4g encoding" but my mobo bios don't have this option. Is a Gigabyte z270x-desingare. Any suggestions?