Use a flat blade screw driver to push the flat plastic part of the mount up under the locking tab on the socket. That is what I had to do to a couple of them to get them to lock in place. When you push the socket down, the very thin plastic of the mount bends AWAY from the socket, so you can't push it down far enough to lock in place. With the screw driver pushing it back against the socket, it should lock in place pretty easy.
Of course, you still have to remove the mount and socket to do it this way.