Hi there,
I have the same mainboard and 8 gpu and use this adapter from Amazon:
( Xhwykzz mit zwei Netzteilen 24-poliges Verlängerungskabel für ATX-Motherboard, 24-polig bis 24-polig (20 + 4), 12 Zoll / 30 cm Schwarz)
I use a power supply for the 6 pin connections on the mainboard and the second power supply for the graphics cards
IMHO I don’t recommend to use separate PSU connect to each GPU (riser and gpu pcie, in this case mobo pcie and gpu pcie). You might damage your gpu and might impact outlet as well…
There is several discussions at reddit related to this topic.
Also, I’m not sure you are using S37 or T37 but, if you are using S37, I don’t recommend to use multiple PSU unless you use risers. This has been discussed at reddit as well.
Have you removed GPU and put them in a different slot? My mobo seemeed to remember where I put cards so I had to make sure I put cards in slots already used otherwise when I tried putting an additional card in and a slot I had used before was empty it did not recognise the new gpu.