When I installed my extra ram I had a similar problem and was able to fix it by clearing the CMOS. What that involves is shutting down your computer, opening it and looking for a battery mounted on the motherboard.
You carefully remove the battery and then replace it. Install all your ram sticks and boot up. Make sure to boot into you BIOS first and check all settings as they will be factory default and you may want to make changes. Finally, boot into windows and see if all the ram is showing.
Since this is a last ditch possible solution and you are playing with the guts of your computer, I cannot stress enough to make sure you are careful in doing this. Make sure you truly power off your box by removing the power cord, press your power on button to drain all electricity, ground yourself by touching a metal part of the tower, pop the battery with a non-conducting item (I used a plastic pen cap), ect