Mob spawn has been nefed so normal dark spawners won't work like they used to in the past. Now, you have to be far enough away to get a mob to spawn, but then the mob won't move and fall into the trap so you have to get close to get them to move. found this post on another forums that seems to have found a workaround. Build the darkroom like normal about 30 levels abouve you. then build a powered rail to take you around the outside of the spawner. you are at one point far enough away so mobs spawn, then the rail takes you close, but outside the walls, for the mobs to start walking. here is the link with some pics so hopefully oyu can make yours work.

http://www.minecraftforum.net/topic/1624564-dark-room-mob-spawner-workaround-146/

I haven't seen your spawner so if yours looks like this already, then i have no idea.