Note: If only one DIMM is used on the computer you are servicing, the card must be installed in SLOT-0 ([a]: lower slot), but not in SLOT-1 ([b]: upper slot).
Please perform the following step as shown in the picture below.
When installing: Insert the notched end of the DIMM into the socket. Press the DIMM firmly, and pivot it until it snaps into the place. Make sure that it is firmly fixed in the slot and does not move easily.