How to farm slimeballs on survival mode in minecraft?

I've attempted to create a slime farm today. I failed. I followed all the instructions that I could find online.

Currently I have 2 rooms that are 16x16 and 6 tall. Every 4 blocks I have glowstone set in the floor for lighting. I've waited 4, 5, 7, even 10 minutes. With 0 spawns. I have made sure that I'm at least 24 blocks away from my farm. I've even tried 50 blocks just to be sure.
I've used the slime calculator with my seed number to find my slime chunks and I know that I'm within the correct area.

I explored the surrounding caves and lit up everything I could find.

What am I doing wrong?

Added (1). Also for those who will say it… YES I'm under 40 blocks deep. My first layer is at 11 blocks and my second is a 17

Guest

New mobs won't spawn in a chunk if those chunks have reached their spawn limit for enemies. Make sure you fully explore all caverns in the area and light them up to remove the chance for other enemies to spawn there that could be spawning instead of slimes.

aozgolo