Impossible error with graphics

I have a maximus 8 extreme board with a 6700k and 2 gtx 970s in sli. One day the one card would no longer show and I tried everything to see why. All my cards work in slot 1 and 4 none in 2 and 3. I can disable all but slot 2 and 3 and put the cards there and get nothing. I can put the cards in slot 2 and 3 and an extra in 4 and it will skip to 4. I tested all the parts and even had the board replaced. All parts work and I even tried another cpu. I can't find whare to look in the bios to see if it detects the cards but I know windows will not detect any card but in slot 1 and 4. It's not a component so what could it be? You would think if it was a windows issue it would atleast work on bios. I'm at a loss of what to do. Please help,

No solution but I had time to browse to MAXIMUS VIII EXTREME | Motherboards | ASUS USA
and look at the board. What I noticed is that Item 7 on that list of features shows that all 4 of the PCIe X16 are not exactly the same, 3 are designated as CPU and 1 is designated as PCH. I think the manual needs to be explored to get the meaning of the differences and whether they can be all used together or the combinations possible.

You would think if it was a windows issue it would atleast work on bios

I'm not yet that familiar with the newer UEFI BIOS and new features but the older BIOS set up the features included on the motherboard and the Operating System used those settings to know how to run devices.

You can run cards in the first 3 in sli but not the 4th. I had it setup with a card in slot 1 and 3 that were in sli and 1 card in slot 4 for physx. Worked fine for over a year and then stopped without ability to be repaired.

Update! With the replacement board it will use the card in 2 or 3 if I turn off all other slots. So all slots do work. This means windowS 10 is having the issue.

Another update...... reinstalled windows 10. Not the issue. Only remaining explanation is that my computer hates me. If anyone thinks of anything please help.

I suspect more that you have a software which incompatible or a driver which is incompatible (old ones) is causing the problem.

Cheers
Tom