Please check, that your memory configuration is such that your two new modules are in one memory channel (i.e. Channel A) and your two existing modules are in another channel (i.e. Channel B)
Another thing you may try is switching the memory pairs around. That is, move the modules currently in Channel A to Channel B and vice versa to see if that improves the memory detection.
Taken from the manual:
Dual Channel Memory Configuration
This motherboard provides four DDR3 memory sockets and supports Dual Channel Technology. After the memory is installed, the BIOS will automatically detect the specifications and capacity of the memory. Enabling Dual Channel memory mode will double the original memory bandwidth.
The four DDR3 memory sockets are divided into two channels and each channel has two memory sockets as following:
- Channel A: DDR3_2, DDR3_4
- Channel B: DDR3_1, DDR3_3
Due to CPU limitations, read the following guidelines before installing the memory in Dual Channel mode.
- Dual Channel mode cannot be enabled if only one DDR3 memory module is installed.
- When enabling Dual Channel mode with two or four memory modules, it is recommended that memory of the same capacity, brand, speed, and chips be used and installed in the same colored DDR3 sockets for optimum performance. For optimum performance, when enabling Dual Channel mode with two memory modules, we recommend that you install them in the DDR3_1 and DDR3_2 sockets.