Boat sliding down trailer when towing

[Cat nip]

What kind of rear straps do you have? And at what angle are they pulling the boat to the trailer? Do they want to pull it up to the bow stop or are they working against the bow winch and pulling it back off the trailer? Any pictures of the back when its on the trailer?

 

[Fed]

When the boat is at rest on the trailer you need to have the roller wedged equally between the top of the bow eye and the hull directly above the bow eye with the strap/cable close to level.
In your case you may need to get a smaller diameter roller to get it to wedge in there because your bow angle is very flat.
I just checked google images to show you an example and all I could find was wrong ones.
No need to tighten the rear straps past a light snug, straps don't stretch so all you will do is bend the boat or trailer.