Question Can't boot mining rig with 2nd PSU ?

[Fish_Fish]

I'm a newbie building my first mining rig. I have 3 GPU's and motherboard on one PSU, and trying to add a 4th GPU on a second PSU.
The second PSU is on with pins bridged. The 4th GPU has power and all fans and LED's are are working. When I boot the system with the first PSU, the display shows just a line of color, then blank.
At the same time it seems that the system is telling the 4th GPU to turn off, the fans stop even though it is getting power from the 2nd PSU.

The PSU's are on riser cards. All is fine with just the first 3 cards and one PSU. If I move one of the 3 PUP's to the 2nd PSU, the problem is the same. So it's not the 4th GPU. Also having the 4th GPU as one of the first 3 GPU's is all fine.
In the fail scenario, when adding 4th GPU on second PSU, I've also tried with fans on that PSU. The fans carry on when the system tries to boot, but the GPU fans still stop and there is no boot.

Motherboard:
ASUS PRIME H310M-C R2.0/CSM
PSU's 2 x 1000w Gold
GPU's are: 2 x 3090, 1 x 3060 ti, 1 x 1660 Super.
I use an adaptor from a PCIe to 4 x riser cards, the adaptor seems to work fine, but even without the adaptor and having just 2 GPU's and one GPU on the 2nd PSU, it's the same problem.
RAM is 2 x 16 and virtual ram in windows is 100.
Any GPU tried on the 2nd PSU is also getting power for the riser from the 2nd PSU.

I want to be adding more cards (if I can ever get any!) but I need to get the system to work with the 2nd PSU first.
I've tried with a different PSU, but no luck. I've tried with a an add2PSU/Sata rather than bridging the pins, but it did not seem to work.

Any ideas???

 

[velocityg4]

Try turning off the power switches on the PSU backs. Flip the secondary PSU switch on first. Wait a few seconds. Then flip on the primary (ie motherboard) PSU. That’s what I used to have to do.

 

[Fish_Fish]

I've tried that several times without luck. It's like the system is telling the GPU on the second PSU to turn off and then deciding it does not have enough power.

 

[Fish_Fish]

I think it was the 4th GPU after all. After unplugging all, it seems to be to one that gives me an issue. Now I have 2 PSU's working with one GPU and system on one and 2 GPU's on the other.