Very few/none monsters are spawning on my 1.4.2 server
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duplicates
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You're not in a mooshroom biome right?
have you got light around? - Is this a vanilla server?
This ticket is incomplete, please provide details about the issue you are experiencing.
The following may help address your problem:
Screenshots of the affected area.
Map file attachment.
Please consider that monsters will not spawn within a certain radius, and may spawn in nearby caverns/underground reducing the count of mobs that are visible.
have you done /gamerule doMobSpawns false
Is the server running a custom map or a automatically generated map?
EDIT: Could you please attach the map
try re-installing the minecraft_server.jar (or .exe)
Your problem is probably the same as this:
https://mojang.atlassian.net/browse/MC-1971
and
http://www.minecraftforum.net/topic/1551224-mob-spawn-fairly-much-non-existant-on-survival-142-server/
Increasing your render distance and/or view-range (in server settings) should fix it.
This ticket has been marked as a duplicate of MC-2536 , in favor of the better description/information provided.
Try changing the difficulty settings of the server. (remember: 0 = peaceful and 3 = hard)