asusx556u :
my ram was first 6 gb ddr3 1x4gb 1x2gb
i removed the 2 gb and installed another 4 gb
my mother board is ASUStek computer INC.
It's because your probably mixing RAM sticks. You cannot mix RAM the has different cas latency, timings and recommended voltage.
More about mixing RAM: http://www.lifehacker.com.au/2010/07/what-are-the-rules-on-mixing-two-different-types-of-ram/