Ram usability

[LaurieM]

Last week I upgraded my computer by adding anew CPU, motherboard and RAM. The job was done by a professional.
The details are:
Motherboard - Gigabyte X58A-UD7
CPU - i7-980X hex core
RAM - G,skill F3-12800CL8T
Operating System - Windows 7 Ultimate 64 bit
Control Panel/System shows the following:
Installed: 12 GB (4 GB usable)
On Saturday and Sunday this had changed to:
Installed: 12 GB (8 GB usable)
On Monday and today this has reverted to 4 GB usable.
Why is this happening and how do I get to use all 12 GB?

[GSKILL TECH]

Did you configure memory settings in BIOS? Reseat the memory to make sure they are firmly inserted. Check the stick in the main DDR3 page, there is a thread about this particular problem.

Thank you
GSKILL TECH