So, one day I made an XP and loot farm by the Youtuber JC Playz out of a zombie spawner. And then I sat at it overnight with a trident killer, and it worked perfectly fine. Zombies spawned just fine until today. Today I was sitting in the farm, and after over an hour, not a single zombie spawned in the farm, which is a dark 8 by 8 area. I left and rejoined the server many times and even left the chunk loaded in and left and rejoined. And after all of that, there is still not a single zombie spawning from the zombie spawner. By the way, the world is is on hard mode with a simulation distance of 4. And I was using a skeleton spawner just 30 minutes before this, and it worked just fine. I also even lit up most of the caves around the the spawner.