In order for the 2x4 GB and 2x2 GB modules to run in a preferred dual-channel config, they need to be installed like in this pic, otherwise they'll only run in single-channel mode...
Rules to enable dual-channel mode
To achieve dual-channel mode, the following conditions must be met:
Same memory size (1 GB, 2 GB, 4 GB, etc.)
Matched DIMM configuration in each channel
Matched in symmetrical memory slots
Check the Memory tab in CPU-Z to see what the current configuration is.