One problem might be the nitwit. I'm not sure if they can contribute to the generation of iron golems because they don't work like other villagers. Also, in the photo, the villager on the right doesn't look like a fisherman...he's not got the hat of that profession. Perhaps he's attached to a different workstation.
One other thing: You need to move your farm up into the air. If you leave it on the ground like that, you will most likely spawn golems next to the villagers on the grass and it might go kill the zombie.