Where Do Slimes Spawn the Most?

Spawning. Slimes spawn in the Overworld in specific "slime chunks" below layer 40, regardless of light levels. They can also spawn in swamp biomes between layers 50 and 70 in light levels of 7 or less. (Levels 50-70 tend to be near the surface.)

Why are slimes not spawning?

Slimes are shy - they won't spawn if other mobs are nearby. Assuming your area is in the correct zone (inside a Slime spawn chunk and within ~40 blocks from bedrock), you also need to check the ravine nearby and light it with torches to keep other mobs from spawning nearby and stopping the Slimes from spawning.

Do slimes need darkness to spawn?

Yes they do. Slimes can spawn in any light level. The only things that affect slime spawning is the height level (i think they only spawn below y62, unless it's a swamp biome) and whether it's a slime chunk. To avoid having other mobs spawn, slime farms are usually very well lit for this exact reason.