After you pull your boat out and travel down the road, all boats have a tendacy to “settle” on the trailer. Meaning that after you winch the boat out of the water and then the pressure the boat puts on the strap and winch, it can have a chance of pulling away from the winch stop due to stretch in the strap. After a couple miles down the road, the boat will shift on the trailer and cause the boat to move forward (if the tongue of the trailer is low due to low hitch) or back (due to the other extreme). Either way, most all boats will shift a bit on the trailer, some more than others. My heavy Ranger 620 moves a bit each trip and I even have the trailer perfectly level during tow.
This may or may not be your problem, but definately something to think about.