Minecraft PE Mob Spawner?

So I made this one survival map on a ported map I found on MCPEUniverse. I made my whole house out of wooden planks and glass. What I was trying to do was to build a mob spawner on the side of my house out of cobblestone and cover all the cobblestone up with wooden planks to make it look better next to my house. However the thing is, mobs aren't spawning whether it's night or day. Any remedies?

I'm don't think mobs can spawn within a certain distance of the players spawn