Posting here just to make sure that I’ve not missed something obvious.
I just bought the NVME hat and have tried for a couple of days now to get Raspberry OS to recognize it. I’m using a Crucial P2 M2 2280 which is in the tested and approved list. The M2 drive works fine in my PC.
I eventually decided to take out a different M2 drive that my PC is running on (Samsung 990 PRO), put it into the hat and it was detected right away!
So I know the board is configured correctly and have seated and unseated the Crucial P2 a few times now but no luck. Is there anything that I’m missing to suggest why it would work with one but not the other?
Well, one Crucial P2 might not be the same as another Crucial P2. The makers sometimes change components, firmware and so on. So this is not surprising. I assume Crucial does test new batches, but certainly not with a Pi5 plus NVMe addon.
I know this is not clear from the docs, but routing PCIe using a FPC cable is not what PCIe was designed for. In fact it does work most of the time, but not in every case and there is no guarantee that it will work in every specific situation. So don’t worry, this is not your fault (but of course your problem).
What you could do: put the OS on a SD-card, boot your OS with the P2 attached and check the logs. Maybe you will gain some more insights regarding PCIe and your P2.