monster spawners dont really work anywhere, you need to have correct environment... a pig won't spawn if there is no grass, I cow won't spawn if there is no light, a hostile mob won't spawn in lightlevel +6... a slime wont spawn in any random chunk...
Slime spawns naturally in swamps, depending upon the phase of the moon, and in specific chunks called 'slime chunks'; in slime chunks slime spawn below LEVEL 40 only.
also mobs wont spawn if you are in a 25 block radius from it.
Nuff~Said...
