You need to apply force upwards as you unscrew to try to get whats left of the thread ( if anything! ) to take. Try tapping a shallow bladed screwdriver under the hexagon section, maybe from both sides to give you that upward force. This exact thing happened to me, its a poor design. I ended up fitting a post with a nut underneath as the thread was lost in the hole. Trouble with that is the hole is too close to the side wall so I had to grind the side of a nut down for it to fit.