Villagers in iron farm stop producing golems after a few minutes.

I've created a 4 module iron farm, and when I first placed the zombies and the villagers, they start producing iron golems normally. After a few minutes of production, the villagers stop producing golems entirely. Strangely, if I kill the zombies, let the villagers sleep again, and replace the zombies once they've slept, they start producing for another few minutes, and then stop again.