I've had this problem with my Windjammer and Equalizer hitch since day one. I think it's the angle of the ball, but it's set where it needs to be to level the truck and trailer.
What seems to work for me when this happens is to chock the trailer well, and then back up the truck very slightly so as to take pressure off of and fully release the tension on the latch. I also make it a point to lubricate the latch and ball before each trip.
2015 Rockwood 2618VS
2016 RAM 2500 Laramie Cummins Turbo Diesel