Assuming that you have a single RAM, motherboard may not be able to detect it if you install it in a slot that is closest to the CPU (Slot A2). You must install it in a slot second closest to CPU (Slot A1). That is by design.
If you do not fill A1 or B1 or C1 before you fill A2 or B2 or C2, the RAM sticks in A2 or B2 or C2 will not be recognized. This is so for my motherboard Asus P7P55D. I do not have info about yours, though. Just an idea.