Iron Farm Help

Hi,

I've attempted to make an Iron Golem farm underground in a server that I play. I copied the one shown in this video: https://www.youtube.com/watch?v=RLrqQY6SfZA

The only changes are 1. It's underground. and 2. The Villagers can't see each other, but they can all see the zombie

There are no other spawnable areas around except for the platform that they are meant to spawn on. I've tested the design above ground on a superflat and it worked fine, it was built in the exact same way as shown in the video.

When my underground variant didn't work, I put holes above each villager's bed (and the extra block of space they have) and ran them up to the surface so that they would have access to sunlight. They have all slept and have worked, but the design still doesn't work. I have also ensured that there are no golems around to prevent any from spawning in the farm.

So as a recap, the villagers have:
Sunlight, A workstation and a bed. They can't see each other but they can all see the zombie, they have all slept and worked and I have seen them run from the zombie. I have read that a golem can spawn when the villagers have slept, worked and are afraid of a mob, so I'm still confused why none are spawning.

Any and all help would be greatly appreciated!

Thanks,
-Will