Not all motherboards support 6 or more cards, even if you have the PCI-E slots on the board. A few settings you can try in the BIOS:
- Make sure you are using the current BIOS for your motherboard
- Change the PCI-E link speeds to GEN 2 or GEN 1
- Enable '4G Decoding' if you have that option
- Disable the integrated GPU and set the Primary display to PCI-E (Usually PCI-E slot 1)
- Disable the integrated audio controller
I have tried all of these, except the '4G Decoding' because it doesn't exist in my bios :/