8 GPU rig: 8th is not powering up (not my first 8 GPU rig with those components)

Hi fellow miners,

So I've been mining for a few years and done several 8 GPU rigs, it's always the same setup:

Asus Z270A
Corsair HX1200i
4GB DDR4
Cheap celeron cpu

The 8th GPU gets connected to an M2 to pci-e converter. Never had an issue with this setup, I usually don't even need to touch the PCI-e gen settings, I just enable 4G decoding and all is well.

Anyway, I've converted some of my 6 card rigs to 8 card rigs recently, and this particular rig is giving me trouble with the 8th GPU. No matter how I plug it in, what settings I change, as long as it sits in that m2 slot with an adapter, it's not getting power (fans don't spin at boot). This is definitely not a Windows issue. Here's the weird part:

If I plug the card in the riser and boot the machine, if I sometimes just slightly tilt the card + riser backwards, fans start spinning (seems random), or just one fan spins. Windows doesn't see the card though. If I just power the GPU without the riser, fans don't spin either (I think that's normal). Here's what I've tried so far:

- Changing riser, riser cable, m2 to pci-e adapter: no difference
- Unplug all cards except this one and leave it in the m2 slot: no difference
- Change all bios settings I could think of (m2 slot to pci-e mode, gen settings etc.): no difference
- Set the card and the riser each with their own pci-e power cable and molex cable: no difference
- Downgraded the motherboard bios to 1009 (same version I have on other 8 GPU rigs): no difference
- Tried bot m2 slots: no difference

If I plug the card and its riser into a regular pci-e slot on the mobo (same riser, riser cable, pci-e power cord), the card get's power and works fine. If I plug the card back in the rig where it came from, it works fine.

This leads me to believe that either the motherboard or the PSU has an issue (I'm tilting more towards a PSU problem as the card is not powering up). Just wondering if anyone else had such an issue before as most things I google are about the 8th card not being recognized by windows, which isn't my case.

Thanks