I had this exact issue a couple of years ago. Exactly the same problem: 16gb installed (and recognised by the BIOS), but Windows wouldn’t use more than 8gb, even though it could detect its presence. And right now, I’m really annoyed with myself for not being able to remember what the fix was. It was… definitely something hardware-related, not software/configuration. I’m off to bed now, so if it comes to me overnight, I’ll pop back in tomorrow.