You can try different riser versions for the non-detecting card.
I hope you have made changes in the BIOS such as changing PCIe lane speed to GEN1 for x1 and x16.
I'll suggest using nvOC Linux distro, as it is tailored for Nvidia mining rigs.
Yeah I had the BIOS set to GEN2, tried GEN1 but made no difference. I've tried everything I can think of but it just won't work.
I think I'll return it and get a different GPU since I know that all of my other cards are working fine on the same riser. Plus I don't like the fact that I have to provide 2 power feeds to the FTW model and the other cards I use only need 1 power feed.