Carnivorous Cube does not work sometimes?

Maybe it is some known behaviour that i am not aware of. But i ate Serpent Dreadseed and cube did not spit it out at the end of the turn! What am i missing?

Carnivorous Cube does not summon anything at the end of turn, only as a deathrattle.

Did you mean Carnivorous Cubicle?

Yeah, this new one. Sorry.