Shadow bug when blocks are stacked

[Creatos]

So I created these models, but when I put them on top of each other the lighting gets weird and dark shadows come on the lid of the block

[KarliosoHD]

I have the same problem with the furnace if I put a block on it the furnace is getting dark inside (if I pit a block at the left/right of the furnace, the furnace is getting black inside too)