Sounds like the ramp angle is steeper than most I load on. That would cause the bow to want to slip under the roller. As maxpower states, just don't back down as far. Try loading with trailer out a bit. You can back in and dunk the bunks in the water and then pull back out. That's what I would try.
Some of my ramps the angle is so flat that I damn near have to have the entire trailer in the water