For some reason my PC is only using 2 out of the 4 Dimm slots. It sees all of my RAM but isn't using all of it. I have 12GB of RAM but my PC and BIOS only shows 8GB. I have an early 2012 Dell XPS 8500 with an Intel Core i5 3350P, 12GB of RAM, Windows 8 64-bit, and an Nvidia Geforce GTX 660. Help would be greatly appreciated.