When I add 5th GPU to rig running 4 GPU's it only reads 3 GPU's

This is my first home made mining rig built primarily for Eth.

I am trying to run 5 EVGA Geforce GTX 1070's, but for some reason the rig hates when I hate the 5th. It runs 4 no problem, hashing at about 120 Mh/s (overclocked). When I plug the 5th in and turn it on only 3 come up as plugged in and sometimes gives a CUDA error in Claymore and all but crashes the rig.

At first I thought it was a riser issue, but if I just unplug any one of them from the mobo it will run 4 GPU's no problem.

Thought maybe the PSU wasn't enough even though I have 1200w so I tried using 2 PSU's, a 750w one just for one card and 4 on the 1200w. This changed absolutely nothing and it still read only 3.

PSU is EVGA 1200w Platinum
Mother board is Asus Prime Z270-A
8gb of RAM
Running on Windows 10

Any help would be greatly appreciated. Thank you.