Other than ensuring your latching mechanism is operating properly you must eliminate any tension that might be on the ball. We always chock our wheels after leveling side to side, if ground is level there shouldn't be much tension on the ball, shift into neutral and raise a-frame off the ball.
If it sticks or on un-level ground we sometimes shift into reverse but with no pedal pressure to help with release. With our towing combo it easy to sense when the ball is in or out of a-frame. We don't make a habit of greasing our hitch ball, we just make sure that it is smooth and the same goes for the area where it is latched. Good luck!!!