"Asus Prime z270-p" & "Sapphire nitro radeon rx 470 4gb" rig problem

Hi everbody,
I have a rig which has "Asus Prime z270-p" & "Sapphire nitro radeon rx 470 4gb". & 750Wx2 Power supply
I can run 3 GPU's at the same time but nothing more, I tried everything and watched every videos on youtube but I couldn't achieve to run 6 GPU's. I see there are some people has the same problem so I create this discussion.

What I have tried until now is,

* Windows is intalled in UEFI
* BIOS updated to the last v0608
* BIOS settings are done
* Above 4G Decoding -> Enabled
* DMI Max Link Speed -> [Gen1]
* PCIEX16_1 Link Speed -> [Gen1]
* GPU's are moded and W/17.4.1 AMD driver

So does anybody can say me, What I forget? Thanks...