Jack,
Steve, the previous owner of the trailer, said to let you know that when you set up the tow ball on your bike, to make absolutely sure you get the very centre of the ball at the same height as your axle. And to do that make sure it is with the bike loaded as if you were riding it. i.e one up or two up.
His reasoning is that if the centre of the ball is above the axle it will try to stand the bike back up mid corner or if it is lower it will try to push the tyre out from under you.
Makes sense from a physics point of view.