Do you have 2 ram modules?
if yes, then check if the second one is placed in the macing color coded slot.
Ram are placed like this: first slot 0 are giving a module, then slot 2. So you see, it has to mach the colors on the RAM slots.
Here is an example. First you use the 2 red ports, then black ports starting with slot1 (the black port closest to the CPU)