Villagers not breeding while standing on a fence post

I made a villager breeder (https://www.youtube.com/watch?v=VTseLpa9Iyc) in the nether but the villagers are not producing babies. They share the carrots I gave them, they display hearts particles, but then they display the "frustration" fire particles and fail to breed. It's 2 villagers and they have 4 beds (2 claimed) and 2 available and within availability/pathfinding.