zombie spawner isnt spawning zombs

[Jenizkitzflare]

hello. im not sure what to do here since all my other mobs are spawning well (mobs before zombie). do i have to make a roof above? do i have to make the area dark? or do i need to use a certain block as the ground?

 

[ulvd]

try make it dark, make sure its closed off and all blocks are blocks they can spawn on