PC not using all RAM

So CPU Z can see all 6GB of ram, even Memtest can see it but in the upper left corner it shows it’s only using 4, also in task manager it shows up as 4GB. I’ve googled far and wide and still haven’t managed to fix it.

Windows 8.1 Pro x64
Asus P6X58D Premium
Corsair CM3X2G1600C8

Screenshot: https://prnt.sc/f9t0w4