System memory is keyed so that it only fits into the slot one way. Because this key is off-center, backwards modules cannot be fully inserted. Check to make sure that the notch in the module's contact area aligns with the slot's key, and press each module into the slot until a click is heard or felt from the latches. Fully seating modules may require a relatively significant amount of pressure.
Our configuration called for a pair of modules in corresponding slots to enable dual-channel mode. Check your motherboard manual to see which slots should be used for this performance-enhancing orientation.
Also note the slot numbers, which are usually written on the board, and compare them to the module installation order outlined in the motherboard manual. This was particularly critical with LGA 1156- and LGA 1366-based motherboards because they relied on a DIMM in the second slot of each channel for termination, though many LGA 1150 and 2011 motherboards aren’t as fussy.
