So I returned the motherboard thinking it must be a mobo fault.
The system would boot (unless only these slots were populated), but anything in those slots would just be ignored. I did the usual trouble shooting, swapping ram around, trying known good ram from my other system etc, but it just wouldn't recognise it. However, nothing gets recognised in slots A01 & A02. I'm using an Asus X99-WS/IPMI, which has 8 DDR4 slots. Hi, I've been having a bit of an issue with the memory in my new-build.