I have 4 RAM slots on my desktop motherboard. I had previously occupied two of the slots with 2 Corsair 1333mhz 2GB DDR3 before I recently added one Team 1333mhz 4GB DDR3. However, the computer won't start with 3 RAMs attached, it will only start with either 2GB+4GB or 2GBx2. Any idea about this?