I'm playing at 1.8 and I can't solve this issue why bats or slimes won't spawn from spawners. I tried to change the biome to swampland and the height is about 40 so what's the matter? Here are the commands:
/setblock ~ ~1 ~ minecraft:mob_spawner 0 replace {EntityId:Bat,SpawnCount:1,SpawnRange:5,RequiredPlayerRange:20,Delay:0,MinSpawnDelay:450,MaxSpawnDelay:600,MaxNearbyEntities:1}
/setblock ~ ~1 ~ minecraft:mob_spawner 0 replace {EntityId:Slime,SpawnData:{CustomName:"Sever Slime",CustomNameVisible:1,Size:2,Equipment:[{},{},{},{id:gold_nugget,Count:1},{}],DropChances:[0.085F,0.085F,0.085F,0.1F,0.085F],Attributes:[{Name:generic.movementSpeed,Base:0.3},{Name:generic.attackDamage,Base:5}]},SpawnCount:3,SpawnRange:5,RequiredPlayerRange:20,Delay:0,MinSpawnDelay:350,MaxSpawnDelay:400,MaxNearbyEntities:3}