Monsters not spawning

Try changing the difficulty settings of the server. (remember: 0 = peaceful and 3 = hard)

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.