You should have received a slip of paper with the cable noting that the connector can be very tight. That’s so that an excellent grip can be achieved. In cases where it’s very tight, I believe it’s recommending that you gently insert and remove a wooden toothpick between the three pairs of gripper plates. A little at a time, as you don’t want to loosen it too much. Just be sure the toothpick is strong enough so that it doesn't break. Not sure about the alternative of using a small flathead screwdriver. It's metal on metal, but then the connector to the socket is metal on metal too.
