Hard to say without seeing it, but if the problem started when you changed the ball, that's a possibility. Could be just a sticky coupler, though. Have you lubricated it ever?
Standard ball sizes are 1-7/8, 2", and 2-5/16 and for tow bars the common size is the 2". 1-7/8 is too small in weight capacity and 2-5/16 is huge, for towing a mobile home or similar load.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL