2x Value Select 4gb 1333 Issue

Hi everybody,



I've recently built a new computer with the following specifics:



CPU: Amd fx4300 3.8Ghz

Mb: Asrock 980DE3/U3S3

Ram:2x 4Gb DDR3 1333Mhz Corsair Value Select (CMV8GX3M2A1333C9)

Vga: Amd Radeon 6700

Hdd: Sata 7200rpm

Power supply: Thermaltake Berlin 630W

Running Windows 8.1 64bit version



By mounting both the ram modules as described on my md manual I get 8gb fully recognized by my bios but only 4 on windows. Using em separately one of the two regularly works at its full potential while the other won't even let the computer boot. No beeps, no usb peripherals leds turn on; the display keeps looking for signals as the computer weren't even turned on.



I brought this faulty ram module to a friend's and have it tested on his computer and, big surprise, it works without any problem. How's that possible? I checked every single slot of my mb and they all work with the first module and won't with the second.



What's going on? :confused:



Thanks in advance.