Your ball socket may be slightly bent if the size checks out and the grease doesn't do much good. Two years ago I had a small utility trailer which came unhitched from the RV bouncing up and down on the frost heaves up in Canada. Got it welded in a remote village, but the welder wasn't too careful and go weld metal part way into the ball socket on the trailer hitch. We had fun everytime we wanted to take the trailer off the hitch, had to bounce it up and down to jar it loose from the RV, so bought a file and did some work on it to make the socket a little easier to remove from the hitch ball. Ended up buying a new trailer tongue hitch coupler at Canadian Tire which solved my problems, so carefully check your hitch coupler for any bends, new ones don't cost much.
