Clear CMOS (either use the appropriate jumper, or remove the battery and replace it after ~10 seconds).
Then attempt to boot with ONE card, directly installed into the physical PCIe x16 slot closest to the CPU. Can you get your system to boot with this configuration?
My system can boot with 1 gpu connected via riser on the first PCIE16x and I already did many Clear CMOS but without any results.
If with these informations you still think it's a good idea, I can test it
If you're certain all individual cards and risers are working, try adding them one-by-one, starting with one connected to the physical PCIe x16 slot closest to the CPU (use this one for video, as well).
Assuming your BIOS is currently at default settings, ensure that you have enabled 4G decoding, and have PCIe link speed to GEN1 or AUTO.