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Created a 50x50 cave with a water shaft in center. The mobs are pushed up my a tripwire/repeater/stickypiston mechanism.

When the cave was at bedrock it spawned, now the top is at y:13

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You're not on Peaceful mode, right? –  fredley Dec 13 '12 at 9:50
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Mob spawning is kinda like quantum mechanics: by directly observing it, you change the result. Mobs won't spawn within a 24m radius. –  MBraedley Dec 13 '12 at 11:33
    
Yeah, where is you pickup spot? Every mob trap needs a spot or area where you stand/kill time doing something so it activates. This has to be neither too close nor too far, or mobs won't spawn. If you've move it up ~13 blocks but didn't move your pickup spot, you are probably too close to the spawning pads for them to be valid spawn locations. –  SevenSidedDie Dec 13 '12 at 16:38
    
Not on peaceful mode (it is multiplayer pvp)/ –  Jonathan Dec 13 '12 at 19:00
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It could be that mobs are spawning someplace else, and filling up the mob limit. Light up the caves and ground near the grinder. –  MBraedley Dec 13 '12 at 19:31
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3 Answers 3

There are a couple of problems you could be having. As has been stated in comments above, very likely many of the mobs are spawning in other areas. Also, if you have moved up 13 levels you have greatly increased the places where mobs can spawn. If you're on bedrock, the area on the surface (assuming you're under land, and not water) is considerably smaller where mobs can spawn, than if you're up 13-20 more meters.

The second issue, is that mob caps are computed globally on a multiplayer server, so if someone pops up on the server, and starts up a mob grinder, basically all the mobs can be spawning in their areas, and not in yours. You have to out compete them on spawning space. 50x50 may not be very large for a well established server.

Finally, keep in mind that most mobs only move if they are with 32 meters of the player. So, water pads like you have are of limited utility if you are far away:

Mob Spawning Ranges, used under CC BY-NC-SA 3.0 license, from www.minecraftwiki.net/wiki/File:Mob_spawning_ranges.png

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Thank you for this! What you said works and makes sense. Instead of using a piston to push them up (i will need to push them up two blocks, which is too complicated of a design). I will create a hole for them, thank you! –  Jonathan Dec 16 '12 at 1:16
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First of all, mobs can't spawn within 16-17 blocks of the player.

I think that the reason that this is not working is because the platforms, they are too small. Maybe a drop trap would be better.

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No, that is a classic mob trap design; the spawning area is fine. –  SevenSidedDie Dec 13 '12 at 16:36
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I have one remark on these posts:

I do have quite some experience building mob spawners and i know for a fact that forcing mobs to spawn in your mob trap (By lighting up caves) is far the most important thing to do to get much mobs in your trap. Your trap might be up to 10 times more efficient. Depending on the size and amount of cave systems, mineshafts, etc. Also, the amount of blocks on wich the mobs can spawn is of course important, but for a good working trap about 800 blocks is more than enough. At least if you did enough lighting..

One last thing: I see a droptrap is suggested. I fully support that

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