GPUs startup lottery?

Have built a 9 GPUs rig (RX 570 x 9). MOBO Gigabyte H470M-DS3H.
8 GPUs with 2 1x4 PCIe cards (with respective risers) + 1 GPU with its riser.
Made all recommended MOBO settings.
But, each time I need to reboot, not all GPUs start…
It ramdomly starts, let’s say 8 GPUs, 6, etc, until after many shutdowns/reboots they all start and show up in the dashboard…
Appreciate in advance your experienced comments.