If you still haven't gotten it un-coupled...ask if the ball has a flat spot on top?
If no flat spot on top, it will either be tough to un-couple or unable to release the ball from the dome of the coupler
As stated in my last post here, that dome's front (towards the TV) goes BELOW the equator to capture the ball and the rear portion of the dome does NOT, but has the latch form the rest of the dome below the equator
Here is a typical tow ball with a flat spot on it's top...that is to allow the ball to move backwards after the latch is released

and here are three different types of latches and their cross section. Note the rear portion of the dome is about where that flat spot will slide past to release from the front of the dome
These do NOT show the ball's flat spot

This one does show the flat spot on top of the ball
