The ball is getting caught by either the pawl (moveable part), or the by the cupped out part at the front of the coupler (most likely). If the ball looks like it is to the extreme front part of the coupler, then back the truck up ever so slightly and easy. If the ball looks like it is positioned to the rear of the coupler, then pull the truck forward slightly.
Usually, if the there is pressure against the pawl, it won't release easily, so I think most times the ball being too far forward in the cupped out part of the coupler causes the problem.
Sometimes if I am too lazy to crawl back in the cab, I can just take my foot and shove the hitch sideways with my foot causing the ball to release.
Chap , DW Joy, and Fur Baby Sango
2006 Ford F150 Super Cab 4x4
2008 Surveyor 263