I recommend you to confirm that your RAM is firmly attached to the motherboard. After that, enter BIOS, system information section and search for some information about the installed RAM. You should see those 8 GB. Enable "Memory Remap" if there is such option in BIOS.
Check this link for further details: http://ccm.net/forum/affich-197079-total-installed-ram-not-usable
Hope this helps