Try a) node away from the hole, jump point at the edge of the hole, and a path node (or zombiepathnode) at the destination point. Then turn on view path. Sometimes, depending on the angle, the path the AI would take from a jump point isn't always logical. In path view, if you don't see path line from the jump point to the bottom floor, they probably won't take it and will only fall by happenstance. Once you've got a destination node at the bottom, play with it's alignment, rebuild changed paths, and see where it has to be.
In my map I had a tall wall that specimen jumped over, from a ramp that leads to their spawn point. I found that when I set the destination nav point away from the wall...I got pathing line. When I put it closer to the base of the wall, I got no pathing line. Even though it looked perfectly logical for them to jump off and land at that pathing point, that's not how the engine sees it.
Hope that helps.