16GB RAM installed in ASUS P7P55D-E LX but only 8 GB usable

so this is the case I have this 2x2gb Mushkin Ridgeback DDR3-1600 CL6 but now I have upgraded my machine to 4x4gb Corsair Vengeance 1600 MHz CL9 but now my machine is using just the 8 GB and the windows 7 ultimate 64bit showing the 16 GB and only 8 GB out of it usable . I did very thing that I can possibly do but nothing is working. I even updated my bios but still I have the same issue . I have even changed the normal"-clocking to x.m.p inside the bios but still no avail . I checked the corsair web page and there in the FQA it say the ASUS P7P55D-E LX is compatible with the ASUS P7P55D-E LX so why is this issue . can anyone here help me on this I would be really appreciating this .

thanks

That is really very much frustrating , I have the same problem with my Gigabyte Motherboard . well when I checked closely I found out that I have not inserted one of the Modules fully . so I suggest that you first check that , see if they have sited fully into the MOBO or just reseat again .

Looks to me it a bad stick of RAM or may be the Motherboard has been defective. In case in case you need to know that I suggest that you try all the ram in good slot,, then pair them up . with the pair try in both pare of slots. You will know if either a bad stick or one (or pair) of slots that are bad. And the main thing , just check the Ram voltage/timings.

Need help on the same issue with my PC with windows 7 it is showing that Installed memory (RAM) 16,0 GB and the 8,00 GB usable. Can anyone explain what is happening here . I just updated the BIOS a few time ago and looking if any other update is available. Please help

The RAM require to be correctly seated Look at the Resource Monitor if the Hardware Reserved 8000MB after that the BIOS is shutting down 2 of the 4 sticks. usually this is a DRAM and/or QPI/VTT Voltage issue.