I had this problem a while back and had to return the motherboard.
I was told It was the 24 pin connector on the board that wasn't connected properly and was a little bent or something.
I couldn't see a problem when I inspected it but that's what I was told by the manufacturer.
But to check other things first I would build it outside of the case. Try one stick of ram and then the other stick/s one by one.
Remove the cpu and inspect the socket and cpu. And then check all the connectors.
Also check everything on the mother board as much as possible.
Much easier to do this outside of the case.