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.
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.
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.
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.