My monster spawner/trap wont work. It is about 25 blocks tall, made of cobblestone and it is really dark. I'm on normal mode and I've tried hard but none will spawn. Heres heaps of pics of my stuff.

![enter image description here](https://i.stack.imgur.com/Od7yE.png)  

![enter image description here](https://i.stack.imgur.com/Pxxjn.png)  

![enter image description here](https://i.stack.imgur.com/g9pqm.png)

I just cant get anything to spawn in it and it has a like mini cave in it and lots of holes for them to fall but they just won't spawn. I've had it for a while and not one monster will spawn in it.