Minecraft – 1.7.4 Problem with mobs spawning

There are two points to remember that may seem contradictory: first, like Nicholas pointed out, you need to makes sure that your mobspawner is the only nonlit space in a pretty big area around you (I think the area's actually about 150x150 blocks), but second, you need a lot of empty space around. In other words, minecraft will choose places that have a lot of surface area (not volume) to spawn mobs in, so if you were to have a lot of open caves beneath your mobspawner, the game will flag that area as more likely to spawn mobs in. You just also have to make sure that it's all well-lit, then you've effectively tricked the game into flagging that area to spawn mobs in, but then conveniently your mobspawner is the only dark area where it can do so.

I would recommend you take a look at this tutorial. You may not want that particular style of mobspawner, but it goes over the prerequisites of any good mobspawner quite nicely.

Yes other mobs will spawn in a slime chunk. The thing that is affecting your spawn rates are the chunks surrounding your spawning chunk. If memory serves there is a sphere surrounding the player. 128 blocks in all directions. Inside of which mobs can spawn. So what is going on here. Is that the mobs you are wanting. Spawn elsewhere. Light up caves in the surrounding area. Or strip mine the whole area. (the method I use.) There is a useful image on the minecraft wiki page regarding spawning. If I were slightly more tech savy I would have provided it.

Happy crafting.