Bow stop question

[Abbeysdad]

So when I load the boat, I'm backing the [bunk] trailer in so the top of the fenders are above the water - this allows the boat to self center on the bunk when I winch her up. There's a fair amount of tension on the strap and I have to two hand the winch to get the bow all the way up to the stop. I hook up the safety and slowly pull her up the ramp and into the lot to put the rear straps on and secure the motor...but here's the problem... Even though I've winched under tension, I SOMETIMES (not always) see that the bow is now several inches (up to 4" or more) back from the stop. I'm guessing that the load is great enough to cause the strap wraps to tighten further allowing the boat to move backward. Clearly I don't want the bow bouncing on the stop so I crank the winch attempting to force the bow closer....without the greatest success.

Any ideas on what I might do to better ensure the boat doesn't slide back on the bunk when I pull out of the launch bay?