If you just bought them, they probably aren't from the same manufacturing lot. Try testing the new sticks by themselves to make sure the board works with them. If they don't work, and the old ones do, you've all the proof you need to return them as defective.
You probably have already tried removing the new sticks and simply re-inserting them, but if you haven't, give it a try.
You didn't tell us the make and model of your RAM, but do check it against Asus' list of qualified RAM. It could be that a non-qualified model may work with a couple of sticks, but it won't work with four.