You probably have your RAM installed incorrectly. I have the same board, and had the same problem. You probably have 1 4GB stick in a "blue' slot, and 1 4GB stick in a "white" slot. They both have to be in blue, or both have to be in white. Once I did that, I had all my RAM available.
My board only has two RAM slots. I was able to get it fixed by switching the RAM around a couple of times (first time didn't work, second time did), so maybe they just weren't seated properly the first 2 times.
Thanks everyone for the responses.