I dug a large room to specifications, 3 blocks tall and 16 blocks wide as well as long. I dug it at 10 levels above bedrock, in a location {(-240, 0) -> (-224, 16)} specified by this slime finder app. I am definitely more than 24 blocks away and less than 32 away from the spawn room, as specified by the wiki here.
Yet, no slimes. I have been sitting in my location patiently listening for the moist slapping sounds of slime. I have gone in to the room every time I heard a phantom slap but still nothing.
Am I just being dumb somehow or do the minecraft lords just hate me?
If it helps/anyone cares here's my seed: 6405460532872068998
Best Answer
Have you ever seen a slime in your farm? If answer is no, good chance you got something wrong somewhere in your calculations.
If you have seen a slime, then very likely there are just too many other places for MOBS to spawn in the 150x150 region around you, that fill up all the MOB slots. Time to break out the torches, and start lighting everything around you.
What I have found best, is to just let it sit for a long time while I do something else in the region, then revisit occasionally. This helps, because slimes spawn more slowly than other MOBS, but they do not despawn like other MOBS, so if you have a good slime spawning region, they will continuously accumulate.... It can get crazy.