They do get stubborn sometimes.
Depending on the slope of the driveway, you may have to chock the trailer wheels or apply trailer brakes with the panic bar on the controller, pull the truck forward to get the ball hard up against the front of the hitch, set the parking brake, and then release the coupler lock.
It also may also release more easily if you try the tongue jack at different heights.
John W. Irwin
2005 Classic 28 "Sabre-Dog III"
2013 Silverado 2500HD Duramax/Allison LTZ
WBCCI #9632, TAC TX-10