Make sure your RAM is seated properly.

Before installing RAM, push the levers on either side of the motherboard chip slots you intend to use into the down position. Grasp your RAM only by the edges--never by the contacts along the bottom edge--and match up the notch in the bottom edge with the bar in the slot. (The module will insert only one way.) Press the module evenly into place, using the grooves that bisect the levers as a guide to center it; the levers should rise and lock into the cutaways at either side of the chip. Sometimes, you'll get the "click" feel of a properly seated chip even when only one side is actually locked in, so make sure your RAM is fully seated in the slot and that both levers fully engage the ends of the DIMM.