R820 Not Recognizing PCIE Device

I am trying to add a video card into a Dell R820.  PCIE 3.0 X16 slot (slots 1 and 6 are specced properly on this server).
I have tried multiple models - a Quadro 2000 being one of them.

I can get this card to be seen in Slot 1 (booted into Debian Linux and run the command "lspci " grep 'VGA'")

I cannot get this card to be seen in slot 6 (or any other video card).
The system is not throwing any errors in the lifecycle controller.

I don't mind buying a replacement Riser 3 but is there any way to determine if the Riser card itself is the problem?
If not, what else might be the cause?

FYI - All slots are enabled in BIOS, BIOS is UEFI, this is a 4 CPU system (so CPU1 is definitely there) - but CPU 1 is used for slot 6 and CPU2 for slot 1 so that is one difference.