Zombie Villagers aren't curing.

I've tried curing a zombie villager at least 12 times, to no avail. I even tried making a new world, but it didn't work. They sometimes can't pick up the golden apple, but when they can, I give them the weakness effect.... nothing happens. They're just zombie villagers holding a golden apple with weakness. The red particle doesn't come up, they don't shake, they don't make the noise, and most importantly, they don't transform.