Are you powering one Delta per rig?
I have the same board and 1 single delta can power up 12 x RX470 (modded and undervolted, slightly OCed).
I suspect not enough power to the motherboard.
During the first boot - get 4-5 cards working first and ensure 4G is enabled on mobo BIOS.
After successful boot up with 4-5 cards, then get the remaining cards's risers inserted.
Yes, one Delta per rig, I wanted to build 2 rigs of 8 Vega64, but it seems I'm stuck. For the moment I have on both rigs 1 x Vega, on one rig it's in the 16x slot and in another is on riser. Still no luck, both are not powering up.
How did you connect the Delta from 12 x Rx470 rig with the Biostar MB? Also with the 160W PICO adapter?