There is maybe a software issue, and very simple fix :
So you have a modded bios , in this case you need to run the AMD/ATI Pixel Clock Patcher" as it was mentioned earlier by tg88
Also if you are asking for help ,the full system spec , used operation system, VGA card driver version would be great to know
Your issue also can be hardware/BIOS settings related ...
What motherboard are you using , what CPU , what kind of vga cards ? Are you sure your motherboard/cpu combination can handle 8 cards ?
Can Your cpu provides enough PCIe lanes for your vga cards ? What is your bios settings for PCIe ? did you set to GEN 1 , GEN 2 , GEN 3, or left the settings on auto ? If you have a option for 4G decoding in your bios settings , did you turned that function on ?
HERE you can find some explanation to understand better CPU - PCIe lanes - VGA cards