Mobs Not Spawning in Your Mob Farm?

Mob farms are structures built with the express purpose of acquiring mob drops more easily and in larger numbers. Sometimes, however, you may notice few or no mobs spawning. There are a few different factors to consider when troubleshooting this.

A tall, gray stone building on top in a Minecraft landscape, surrounded by grass, sand, and water.

Location

The purpose of a mob farm is to provide a large area that is a valid spawn point for the intended target, and to kill those targets quickly. Due to the rules Minecraft applies to spawning mobs, this makes the location of your mob farm very important.

Farms placed on the surface can provide good drop rates during the day when it is one of the few spots of dark ground, but has a sharp drop in effectiveness during nighttime, as mobs are able to spawn most anywhere.
Farms built underground have a time-independent drop rate and remain within your personal spawn range while you’re mining, however, their effectiveness drops when there are nearby caves that provide alternative spawn points.
Farms floating high in the sky are also time-independent, and benefit from being far away from caves, however, building one in survival can be quite dangerous and, due to their height, they stop working completely when you descend.
Farms constructed under an ocean also work both day and night, as the ocean limits the spawn locations to open areas underground, except for drowned, which spawn underwater.
Superflat worlds provide higher spawn rates due to missing air pockets underground that would otherwise provide alternative spawn locations.
The Nether is also difficult to farm, as water evaporates and most mobs are immune to fire, reducing the number of functioning farm designs considerably.

Farm Design

Monsters require a certain set of conditions to be met to spawn. The block directly below it must have a solid, opaque, top surface. This includes top slabs, upside-down stairs, and others. Exceptions to this are soul sand, mud, or a slime block. This block also cannot be bedrock, barrier, or a command block. The mob’s collision box must not collide with any liquid, meaning mobs cannot spawn in water.

The block above the spawning block must either be transparent or air, and the mob must be immune to the damaging effects of the spawning block or the block directly below it.

For example, if your mob farm was built out of magma blocks, only mobs immune to fire damage will spawn.

Performance

Minecraft has a limit on the number of hostile mobs that can spawn in loaded chunks, so it’s a good idea to make sure all of them spawn in the farm. This can be done by clearing out the entire chunk where your mob farm is and lighting it up with torches.

Also, keep an eye on your server’s TPS, as this impacts how often the server spawns mobs. When the TPS falls below 20, the server may skip spawning mobs to save server performance.

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