HP Microserver Gen8 - 8GB installed, only 4GB usable. How to get all 8GB working?

Hi all. Here's the scenario:

* There's 8GB physically installed, as two 4GB modules.

* Both modules work.

* The BIOS recognises 8GB.

* POST says only 4GB is installed. -- http://i.imgur.com/j5HFcpo.png

* Windows recognises 8GB, but says only 3.9GB is available.

* I'm using Windows 10 x64. The OS isn't limited to only 4GB in startup options.

* The system supports 8GB

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Any suggestions would be welcome. I really am stumped on this one.