[JRTJH]

Some people assume that all trailer balls are the same, they aren't. In some applications, the brand of the coupler, the way it's built and the type of trailer ball make a HUGE difference. Looking at this cross section of a coupler, you can see how the ball "fits into the cavity" moving forward slightly as the fingers grip under the ball. Notice how the coupler cavity "sort of grips under the ball in front as the fingers push against the ball in the back?

Now, look at the three shank designs on these photos of trailer ball. You can see how some would "bind on the fingers" if the front lip of the coupler pushes against the shaft, forcing the ball back tight against the fingers if the rear of the trailer is higher than the ball. The same can happen if the rear of the trailer is lower than the ball, by pushing the fingers against the rear of the shaft, forcing the front of the ball tighter against the coupler lip on the front part of the shaft...

Sometimes, just changing the trailer ball to a different shaft machining will make all the difference in the world in how easy it is to uncouple and/or to lock the fingers behind the ball while hitching...