Sometimes people take too much weight off (all of it) the hitch. If truck and trailer are not on the same plane... . ie trailer and truck are not level side to side to each other it will bind the pin enough to not unlatch. Putting a little more weight on the truck usually does the trick, in addition to as already stated, backing slightly,or allowing truck free movement to find its spot. When its giving grief, it helps to have someone pull some tension on the release while you try a couple of these remedies.