The two small pins on the bottom of the bulb must line up with the small holes in the base of the socket. This can be tricky since you can't see it once the bulb is inserted. Make sure their nothing already in one of the socket pin holes (like part of the old bulb) and the note the socket hole locations and (lightly, using your finger tips) fit the bulb up into the socket and wiggle it until the pins and holes align. It does fit, but sometimes it is not without a challenge.
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The two small pins on the bottom of the bulb must line up with the small holes in the base of the socket. This can be tricky since you can't see it once the bulb is inserted. Make sure their nothing already in one of the socket pin holes (like part of the old bulb) and the note the socket hole locations and (lightly, using your finger tips) fit the bulb up into the socket and wiggle it until the pins and holes align. It does fit, but sometimes it is not without a challenge.