Bunk trailers have a tendency for the boat to 'pull-away' from the bow stop as the boat/trailer are pulled up the ramp.
This happens because with the trailer on the ramp and the boat still floating, the hull contacts the forward part of the bunks. As the boat comes out of the water, 'geometry' says that the bow/eye pulls away a couple of inches as the hull rests on the entire length of the bunks.
You might be able to remedy this by pulling the boat up the ramp just a couple of feet and re-winching.