I made a spawner trap in the sky, and it's not working. Anyone have any idea why?

Here's a picture of the interior (sorry for the crappy size, I couldn't make it bigger). It's like that in all four corners. The torches are there for light for the picture, it's normally completely dark. Here's a picture of the outside. The idea is that they spawn on the blocks in the first picture, then step off of the blocks into the water, which pushes them to the holes, then they fall to their death. I made it, put it on easy mode, and waited a few nights, and nothing came out. What's wrong with it?