HI I had exactly same problem as yours. i am using an American megatrends BIOS and it has only 2 RAM slots. I inserted 4x2 gb RAM. Firstly, when i used to look at my system properties(by right-clicking MY COMPUTER> PROPERTIES) i used to see this : MEMORY = 8GB(3.90 GB usable). I tried changing the the Positions of the RAM sticks, inserting new RAMs and changing the GPU. NOTHING WORKED !! i tried asking all the LOCAL COMPUTER TECHNICIANS about this problem but no one helped me... Finally, it was time for me to decide something by Myself
. i started the pc, entered the BIOS, got into 'CHIPSET FEATURES', ENABLED THE 'MEMORY MAPPING' , ENABLED ALL THE PORTS, and finally, i used the option 'USE OPTIMUM SETTINGS' on the main BIOS screen. Now, when i start my pc and SEE the properties of My Computer, i see: MEMORY: 8 GB. now, u must be thinking what proof do i have that MY PC is making optimum use of RAM. Well, i may say that My PC & GAMES are working way faster than they used to be. As you are using an ASUS BIOS, i dont know much about ur BIOS. but i ASSURE u that ASUS BIOS are way better than AMERICAN MEGATRENDS and so, u can easily change the settings of ur BIOS. Just look for the Following Options and change the settings. 1) MEMORY MAPPING- enable it ! 2) USE OPTIMUM SYSTEM SETTINGS- Select it and select 'OK' I hope this works and it should