I had this memory problem with my i7 build- the 6 gb memory only showing up as 4. Tried many things, and searching forum, it seems that in some mobos, the things to plug the cpu cooler may be too close to the memory, and in effect, with inconsistent seating of the cpu cooler, may bend the memory slots such as they don't work too well- mine started detecting 6 after I changed cpu coolers, so I haven't touched it since, but that might be it- maybe try loosening the screws holding down the cooler a bit?