I've tried building a mob spawner in my house. It's on cobblestone, completely made of cobblestone, 2x2 tunnel width and a 32x32 at the top, 5 blocks high, completely dark. Nothing spawns up there though. How can I get it to work?
It could just be bad luck. Your area has the same chance to spawn mobs as every dark 32x32 section of the ground (if it is night) or cave. To compound this, there is a spawn cap of around 80 hostile mobs, so if they already exist in caves or outside, no new ones will spawn anywhere until the amount is under that limit.
To deal with this, light up your caves and surface as best as you can within 128 blocks from your grinder to keep the amount of mobs outside to a minimum. You can also increase the amount of spawn space available by building a few more floors for your grinder.
Also, make sure that you are not playing on peaceful difficulty.