Rafael Santos

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.

Ok I found a solution.
Acording to this thread "mob spawn rates are tied into the number of chunks loaded in memory"
http://www.minecraftforum.net/topic/1551224-mob-spawn-fairly-much-non-existant-on-survival-142-server/
So just increase your render distance and/or view distance (if server) and mobs will start to flow down =P
Not sure this is intended though.

Hi I'm having the same issue.
I have a mob trap starting at Y=95, all caves around have been lit so its the only possible spawning space.
When I enter the game some mobs spawn but after that it almost stops, only 1 or 2 mobs spawn if I'm lucky.
Using Rei minimap I can see that there are no mobs anywhere else.
However if I move near chunks that have unlit caves monsters start to spawn on those chunks.
I've had this trap since 1.2.4 and until the last update it worked fine.