After my success with the NVIDIA card, I decided to try it in each slot
I found that it wouldn't work in slot 3, that is the middle 16x slot.
I tried removing and re-inserting it a couple of times and I inspected the slot with a torch and couldn't see any problem
Next I tried removing and re-seating CPU #2. After doing this, the NVIDIA card in slot 3 is working.
Are there any diagnostics to confirm CPUs are correctly inserted? Everything else I did on the CPU appeared normal, for example, I had exercised all cores and RAM without problems.
Is there any technique to follow when inserting the CPUs to avoid issues like this?