have you used the PCIe x16 port on MB when plugging in your risers or are all of them plugged in small x1 ports?
I use the PCIe x16 for the main GPU and all the rest are on the x1 ports
When I was installing them 1 by 1 I was using the PCIe x16 port
When you tried to add that GPU one by one, don't use the same port for all GPU. Add every GPU on the different port (
each GPUs must be adding on the different port) and also use the different riser while installing each card.