Minecraft: Mobs disappear when i get close?

I play minecraft on XBOX 360 and have built an xp farm. It uses a water canal to bring the monsters down and to the place of killing. It works just fine, except that whenever i get close to the area where i can kill them they disappear. Sometimes this doesn't happen right away, but only after i hit them once. I can't anything out of this, and am really kinda bummed.

I tried covering the tunnel up so everything is dark except for the final area, but this had no effect. They even disappear at night too.

Guest

I have the same problem too! Lol
But my brother told me that I will never get a chance to kill any of them, they just don't die. I wish I can disappear like them when someone is about to kill or hurt me, Lol

Aria

That means they don't like you killing them

Colin

I think it has something to do with the spawner. Only a certain amount will spawn when a player is x amount of blocks away. Try and see if you could drown them with some crazy mechanism and collect the items like that.

Nicholas