In some plains biome village, there's a house with a blast furnace inside. Usually the unemployed villager walks inside the house to reach the job block to become an armorer, but instead the villager just looks at the block and cannot transform. This is due to an cobblestone arch which covers the blast furnace and prevents the villager from reaching the job block. Even if you push the villager exactly next to the job block, the villager won't transform and keeps looking at the block. However, as soon as you break the cobblestone arch or just the single brick block on top of the blast furnace, the villager transforms.

So the armorer's house in the plains biome village needs a tweak.

Wanna see this yourself?

Seed: 701001644968593818

Village Coordinates: 186.564  72.00000  -349.569