Can't get 5th gpu on riser to be recognized, but CAN on motherboard.

Hi All,
I'm really at my wit's end here. I'm new to mining. I Have built a rig with 5 EVGA gtx 1070s SC.

I am using an ASUS-P6T7-WS-SuperComputer Motherboard.
EVGA 1000W SuperNova G2 PSU
6 GB Ram

I am running ethOS for mining.
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My problem is that I simply cannot get 5 cards to work on 5 risers. The only way I can ever get 5 cards to mine on my mobo is to put 4 on risers and 1 on my MOBO's PCI-e slot and I realize that I should not have this one card on my MOBO (though I'm not entirely sure why, I realize the proper thing to do is to have them all on risers)

Every time I put a 5th card on a riser, the 5th card, and sometimes even the card next to it have their fans twitch at start up and they don't get recognized by ethOS. I can do a sudo hard-reboot and often the 4th card will come back and get recognized but never the 5th one.

I have tried nearly everything I can think of. I've checked to make sure everything was connected correctly and firmly nearly 100 times. I've changed out to entirely new risers and cables , swapped card places, Tried different ports on the PSU to make sure one just isn't burnt out. Nothing seems to work. The problem always returns as soon as I add a 5th card to a riser.

Once I take that card, or any card and put it back directly into the MOBO , the 4 cards connected to the riser run, and the one of the mobo runs no problem. I know it can't be that I can only run 4 risers with this mobo. Can someone please give me some help. ?

Thanks for your time.
Mike