Got any tips for getting the last slot working?

[HashZilla]

I have two main boards from different mfr's and each has difficulty filling the last PCIe slot.

The last RX470 only works at 300Mhz GPU core and hashes about 4MH/s, and won't take settings from Wattman/Wattool.

In both cases, I tried a different card, riser, cable, and the same items work properly in another slot or MB.

I tried switching BIOS to PCI Gen2, re-flashing cards, re-installing from bare metal.

Any suggestions?

Background:
ASRock Z97 Anniversary
Celeron 1840
Windows 7 64bit
Sapphire RX470Nitro 4G
USB 3.0 Rev6 risers

ASRock Q1900M
Celeron J1900
Windows 7 64bit
Sapphire RX470Nitro 4G
USB 3.0 Rev6 risers

Thanks for looking this far.

[adaseb]

Plug a monitor into one of your RX cards.

If you want to use 6 GPUs, you need to upgrade to WIndows 10.

[HashZilla]

I am trying to get only 3 (J1900) or 4 (Z97) GPU's working and I always l have the display connected to the first RX that I install.  This has something to do with the MB.

[methylminer]

Plug the monitor into the display getting the low hashrate.