Windows 7 won't see or you all of my RAM

Hello there,
for some reason my OS won't see all 6 GB of RAM i have installed on my motherboard all of the time, sometimes when i look in control panel > system is says i have 6 GB installed and other times it says 4 GB

At first i was told to check all the sticks (3 x 2 GB) were working as well as checking each RAM slot on the motherboard, after swapping all the sticks around i found they were all working including the slots and my OS was seeing 6 GB (Y) Great! But the very next day it was only seeing 4 GB again

I'm not sure what to do next, i've looked in the BIOS and moved the sticks round into different slots to see if any were "unhappy" in certain ones but with no sucess. The only thing i havn't tryed is updating the BIOS as a friend told me this was a last resort.

Thanks alot

Oli

P.S.

All parts of my computer are brand new 1st hand and are in no way damaged

whs said:

You have to do "Memory remapping" in the BIOS - if your mobo supports it.

this.

update bios, check this setting (and then maybe run memtest, too).