TL;DR
If you have 4 memory slots and only 2 DIMMS, be sure you are using the 2 slots preferred by your motherboard manufacturer. I have 4 slots (A1,A2,B1,B2) and couldn't get expected speeds using A1 and B1 but got the speeds with no trouble on A2 and B2 as the mainboard FAQ (and, presumably, manual) suggest. Who needs logic?
Preface
So I built a new PC recently and it was tons of fun. My sons helped me. I mostly went off the cuff (I've built a few before) and referenced the manual a couple of times. The first mistake was putting the M.2 SATA drive in the NVME slot (which doesn't work)...but moving it to the other M.2 port works. The second error wasn't as obvious, as the PC booted mostly just fine.
Memory
The memory I purchased (2x8GB sticks of Corsair Vengeance LPX cmk16gx4m2b3200c16) wouldn't run at the expected speed. It defaulted to 2133, but when I bumped up the the XMP 3200, it would boot three times before fail-safing to the 2133 speed. I updated the BIOS before even starting, so that wasn't it. I tried various settings and was finally able to get 2800 working mostly, with 2666 being completely stable. This wasn't terrible, but if I wanted slower memory, I could have saved $20 or so and gotten a cheaper kit. After reading into custom timings and memory training (woah, that escalated quickly), it dawned on me that memory placement *may* matter. I had put the memory in A1 and B1, which made sense to me. When I glanced at the FAQ on the website, though, it said to use a2 and b2 if only using 2 of the 4 available DIMM slots. I only decided to try this because I'd seen reports of 4-stick kits not achieving the same speeds as 2-stick kits. FINE, I'll try. Bam...worked immediately, first boot, with default 3200 timings -- CPU-z verified speeds. So far, my minimal benchmarks haven't really shown much gain, but I slept a little better last night knowing my speeds were as expected.
Takeaway
I was muttering bad things about Corsair when the memory didn't perform as I expected, even though it wasn't a verified chip for the motherboard (ASRock AB350 Pro4). Turns out the problem was mine, though I'd lean a little towards blaming ASRock over Corsair since the board prefers the non-logical 'a2' and 'b2' slots. For the record, I'd imagine the manual would have the information if I'd read it more closely. If your memory isn't working as expected with current BIOS updates, verify the memory chips are in the correct slots (if you have 4 available slots and 2 dimms). Your manual or manufacturer website should help!
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