Push down on the side of the coaming…
... is the crucial trick, or on the edge of the hull. That rolls the boat under the victim, past the stability point and makes it pop the rest of the way on its own. The hull works in much the same way it does in a regular roll when you get past the stability angle.
I also find that it's best to push down toward the ~back~ of the side of the coaming, because that's where the center of buoyancy tends to be if you have pressed the victim's torso onto the back deck, which you should have. I have seen difficulty when pushing more toward the front, and I believe that is because you are yawing the victim's boat quite a bit in addition to pushing it down.
When you do get it, it will be almost effortless, something that even a lightweight person can do with a heavy victim. Just make sure to hike out and commit your own body weight to the downward push on the coaming.