I had the same problem two days ago when cleansing my computer. I pulled out everything from the board including batery (to clear CMOS), and when I installed everything back again and started the computer, bios shows that memory is 4 GB but just 2 GB enabled.
I got it fixed by pulling out memory and installing it back while shorting the "clear CMOS jumper". I have P55A-UD3P with F14 bios, so I guess it may help.
Don't forget to umplug the power cord before doing this.