If you have 2 modules, they should be installed in slots having the same color. According to the SPD tab in CPU-Z , are both modules identical?
This is not always true.
You will have to consult your the manual for your motherboard. If you don't have one, you should check out your motherboard's manufacturer's site. They should have it.
Here's the manual: http://dlcdnet.asus.com/pub/ASUS/mb/socketAM2/M2A-VM/e2975_m2a-vm.pdf
Accoarding to it: