Yesterday 12.26.17 my zombie and skeleton spawers were working. Today 12.27.17 I got on to play and the spawers had no zombie or skeleton on the spawner. There is no light in the room with the spawner and as i said they were working yesterday. My world is on survival and and was both normal and hard at different times to see if it would work. Also mobs were spawing other places, including in my village that has a good amount of light so mobs will spawn.