Problem with mob farm

[darkpouetman]

I just build this mob farm and nothing is spawning inside, did anyone else had the same problem? I always make sure to be at least 24 blocks away and I know normal mobs can spawn because some spawned at the bottom of the tree-house. Any ideas? I would try cursed earth but I didn't find a division sigil anywhere and I don't have enough thaumcraft setup to fight the wither yet.

[3Divi]

clear the blight, regular mobs won't spawn on cursed land biome.