No Mobs Spawning in Mob Grinder

I used this tutorial to build a mob grinder.

However, my grinder is not spawning any mobs at all.

A little background info:

1. I built pretty much the exact same mob grinder at Y=90 with a drop to Y=63. It spawned some mobs, but not many. I also noticed that spiders tended to block the exit of the spawn room and also climbed up the walls of the tower.
2. I googled a bit and found two solutions. Using half slabs to not have any spiders spawning or to make the tower wider than the exit of the spawn rom (spawn room exit 2x2 followed by the 4x4 tower). I preferred the second option as I would still like to get drops from spiders.
3. So I built the tower 4x4 wide and then had to realize that suddenly no more mobs were spawning. So I googled a bit more and read about spawning behavior and everything. I figured that probably there were too many caves around and/or I was too far away from the spawn room I built.
4. So I decided to follow some advice I found and build my tower higher up so that there are no spawnable areas close by. I then went up and built the exact same design at Y=200, leading through the existing spawner and dropping to Y=63.
5. I also built an AFK room at Y=175 to be within 24 to 32 blocks of the spawn room, but still no further than 128 blocks of the bottom, so that mobs don't despawn along the way.
6. Not in peaceful, I am playing on normal difficulty
7. The spawning platform is 20x20. Does this have an impact? The lower platform is also 20x20 but at Y=90 and there is nothing spawning either.

Unfortunately, I am still not getting any mobs to spawn at all. Even after waiting about 45 minutes there are no drops in the chests or the hoppers and the spawn rooms are completely empty.

Am I missing something? I would appreciate any help you can give! I attached a few pictures for your reference. Please let me know if you need any other info.

3 Answers

I think the issue you're running into is the size of your platform is very small, while you're built very high up in the air. How spawning works in Minecraft, simplified:

1. It picks an X/Z location to spawn in
2. It picks a random Y location from 0 to the highest opaque block above it. So if the highest block is at y=15, it picks from 0 to 15. If the highest block is at 255, it picks from 0 to 255.
3. Look at that X/Y/Z location to see if its a valid spawn location (has a flat surface underneath, is sufficiently dark, the player is between 22 & 128 blocks, etc.)
4. If it is, it then spawns 1 mob at that location, and 2-3 of the same mob nearby (using a similar, but simplified algorithm)

What this means practically is that the higher you build your mob spawner, the more spawning spaces you need to get good rates. 400 spawning spaces at y 200 is going to be too low to get many spawns, especially if you're checking it fairly regularly to see if anything spawns.

Other potential issues:

1. The farm you're using relies on mobs walking into the water themselves. They'll do this if you're standing between 22 & 32 blocks from them, which it sounds like you're doing, but it's fairly slow, which will contribute to you feeling like you're not getting drops.
2. The farm design you're working with is very old (from 2015), and mechanics have changed a lot since then. In particular, water pushes mobs more slowly the longer the stream, and if it's pushing them at the 8th block of flow, it won't push over the edge.
3. While mobs will instantly despawn if they are ever >128 blocks from a player, they will also be chosen at random to despawn if they're greater than 32 blocks (this number may not be accurate, it might be higher) from the player. The chance of a mob being chosen to despawn increases the farther they are from the player. So, the mobs you're dropping may be despawning as they fall down the tube.

But, the main issue is the number of spawn spaces.

For a more in depth explanation of the mechanics, and a more up to date farm design (though it does still suffer from issue 2 described above), see this video:

I think your issue is the lighting of surrounding area

The spawning mechanism of Minecraft is simple:

No mobs spawn within 24-block radius and all mobs instantly despawn when they are further than 255 blocks away from the nearest player.

To increase the drop rate, light up every block(light level>7) within 255 radius around the afk zone. Be sure to check all caves underground, they are your biggest enemy when building mob farms.

The reason is because mobs only spawn from 148 blocks away from caves, if it is closer, the mobs will spawn in the caves instead of the mob farm. There's a solution to this. You can build the farmer on the ground but only if you lit up all the caves and dark spots in a 148 blocks radius. You can do one out of the two, either build it high up or build it on the ground and lit the caves in a 148 blocks radius, it's up to you. Wish you good luck!