Mob farms do not seem to be working properly, and seem to be spawning virtually nothing in most instances.
For this, I have screenshots of what I've tried:
#1 standard, 2 high spawner, with a water drop for the killing blow
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After an hour of waiting AFK, nothing spawned. I took in account spawn limits, and did my best to light up any caves in the area. Still, nothing spawned.
#2
My next attempt was to raise it from the ground. These screenshots should be self-evident, but it appears, that even with 4 blocks above the floor, mobs prioritise spawning on a flat open plain, with no sides, or something with a more rigid bottom. Instead of spawning within the spawner, the spawned on top of it. When I added layers to see if walls were the problem, they again, went to the next layer.
Here's a series of screenshots to demonstrate:
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Something isn't working right. Not sure what it is.
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Forgive me if the answer is in the screenshots (wifi is horrific for me tonight so after 5 minutes I've given up waiting).
How far away are you from the mob spawners you have made? There's a number of significant differences between Java and Bedrock so if the mob spawners and AFK positions you made are based on Java mechanics that can sometimes explain the disparity. I can't remember the spawn radius of the top of my head but its much more restrictive when compared to Java.
Another common cause is the "global mob cap". Bedrock has a global cap of 200 loaded entities. This includes almost all entities both hostile and neutral (and in 1.14 even armour stands), so if you have, for example, a chicken cooker with 100 chickens, a villager trading hall with 40 villagers and a cow farm with 60 cows, you will prevent all other mobs from spawning. Thats an extreme example but even half of those numbers with some fish, mob spawns underground and its gone.
That cap applies to all mobs within ticking distance of you (so for a ticking/simulation distance of 8 that's 128 blocks in every direction. One way to test this would be to build just under height limit somewhere ~100 blocks from your current main areas and see if that's any better.
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You have to be away at least 24 blocks for mobs to spawn
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It was more the max I was referring to. I believe mobs spawn between 24 and 54❓ blocks from you, so your farm needs to be within that disk.
Mob density also comes into play but I know very little about the density caps.
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@IonEcko
Quick edit: I sat AFK creative floating near the 'spawner' while watching Youtube. My position should have been fine to keep it active. Just wanted to confirm that.
Thats kind of why I was trying it at different height levels...for mob cap reasons. Its why I moved from the ground spawner, to one that moved higher up through the air. I was trying to escape the mob cap.
Though, I have realized my biggest mistake in the rush...I forgot to light up the tops of those higher platforms to ensure the mob cap would stay limited at higher altitudes, and to ensure they spawned UNDER the platforms, and not on top as they had been.
They seem to prioritize open air over anything with a roof at least 2-3 blocks high. But they do eventually spawn beneath there after a time. Found that out this morning after waking up (I had been testing this til 2 a.m - which probably didnt help things).
So not a bug, I guess, but a definite tweak to the way mobs spawn would be nice to see. Honestly just more parity with Java in general would be nice to see, haha. But thats another tangent.
Still gotta figure out if trap doors still work for creeper farms though, as this was the entire point of my experiment, and I keep hearing mixed things online.
Closing off, I should probably close this bug report. Still would like to see tweaks to mob spawning behavior on bedrock, but I cant say this is a bug any more.
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I have learned its beneficial to set your world difficulty to peaceful then back to your preferred difficulty when your about to afk. Just like Ecko said the mob cap in the world fills quickly and especially if you are playing multiplayer.
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@Max H
....
That probably would have made testing so much easier. I don't know why I didnt think of that. Guess I was trying to keep things 'natural' but even so, that would have worked and made testing much faster.
I feel like a dumbass lol.
GoldenHelmet
Bedrock Edition has separate density caps for surface and cave spawns. The monsters spawning on top of your farm rather than inside indicates that the cave cap was full. Being at a different height does not change the cap, but being >54 blocks above ground will prevent new spawns down below. Unfortunately, monsters won’t despawn from caves unless you light the caves first. What Max H said about cycling peaceful mode is a common workaround for that. In 1.16 though we will have instant despawning like Java (though at a shorter range), so this kind of farm will be much easier to get working.
This ticket will be resolved as a duplicate of MCPE-21856.