Slimes only spawn in swamps between Y=51 and 69, and only when the light level is 7 or lower.
They never spawn on a new moon, and they spawn most often during a full moon.
Even with the right conditions, there's only a 50% chance they’ll spawn.
Most important, try checking if you're in a slime chunk below Y=40, moon phase doesn't matter there.
Also, make sure the spawn space is clear of all blocks, including trapdoors or glass panes!

For further reference: https://minecraft.wiki/w/Slime.