IMHO, the secret is ramp angle, as in Graham raising the tongue with the tractor. I remember an add from the 50's for a trailer with the tongue pinned under the trailer frame back in front of the axle. The frame locked onto the tongue for travel and unlocked at the ramp letting the frame rotate to an angle to let the boat slide off. There was a photo of launching a skiff with the rear wheels on a 3' high culvert!
If the trailer is too flat with just the rims in the water at a given ramp, changing supports (trough, roller trough or whatever) won't allow a launch. Two options to consider would be a snatch block under the boat for the winch wire to pull the boat off, or a 'brake tongue' to increase the angle.