Fixed it - I unplugged all the memory sticks and swapped them around, and now it displays 32 GB

I have installed 32 GB of RAM in my R640, but the BIOS only shows 16 GB - 16775168 KB - I've updated the BIOS to version 1.09.1808, but this has made no difference. The RAM is 8 sticks, each of 512Mx72 DDR2 PC2-5300.

How can I get the BIOS to recognise the full amount of RAM?