12GB of RAM installed, only 8GB available

[hatcat]

Although there are 12GB of RAM install on my motherboard, Windows 10 is only reporting 8GB, and the BIOS cryptically reports only four of the six memory slots are enabled.  How do I enable the other two memory slots?  Is this an error or a fault?  Do I need a new motherboard?  Is it a BIOS setting that I can't find?  How could I find this out from documentation?

Board: Gigabyte Technology Co., Ltd. X58A-UD5 Rev 2.0
Bus Clock: 133 megahertz
BIOS: Award Software International, Inc. FA 04/20/2010
Windows 10 Professional (x64) Version 1511 (build 10586.218)
Boot Mode: BIOS (Secure Boot not supported)
2.85 gigahertz Intel Core i7 930

8192 Megabytes Usable Installed Memory

Slot 'A0' has 2048 MB
Slot 'A1' has 2048 MB
Slot 'A2' has 2048 MB
Slot 'A3' has 2048 MB
Slot 'A4' has 2048 MB
Slot 'A5' has 2048 MB

Thanks in advance,
Guy

[shadowsports]

Is the memory all matched?  Same brand, spec, speed, etc...  Have you tested each stick individually or in pairs of 2?  At the very least tested the 2 that don't appear to detect in BIOS?