I second most of what others have said here. Takes some time to get the hang of it. You have already figured out that the correct height of the kingpin and hitch plate is key. One thing I learned (the hard way) is the trigger latch inside the hitch plate opening MUST be in the proper position or it won't cause the jaws to latch. Finally figured it out one time and used a big screwdriver to push it into place. NEVER put your fingers in there to mess around with anything though!
As CORattler said, the secret to easy unlatching is to raise the trailer just so it starts to slightly lift off the hitch plate and make sure there is no tension on the latch by allowing the truck to very slightly move back (I just usually put it in neutral or reverse and then neutral without even moving) It doesn't take much tension to make the release handle hard to move.
Be sure to put a hefty padlock through the release handle opening when hitched to prevent anyone from releasing the handle when you don't want it to be!
