The picture helps a lots to understand your problem. painting with skid / anti slip paint should help. I have a 16 ft trailer with side bars and ramps. So far I have not ran into the problem you have.
my first post, so maybe I misread something. I though the question was about loading ATV onto trailer. It is a nice hook on truck.
I would ask, what is your loading gate or ramp made of , wood, metal.