Wait, does the towball move in the attachment? It should not. I did have some Westfalia towbar in my Civic and it was hard to get to tight because it was based on some eccentric tightening principle but all others have been very tight. The one in my other car is Bosal and it sits so tight that it does not fix itself if there's even one sand grain between it. So it needs to be clean and then just insert, and it clicks, automatically tightening itself: