exp farm help

[wfs0801]

I've built a small canal type exp farm on my server for levels so I can try out enchanting. The monsters basically get funneled into a bottom 3x3 hole, where I would ideally come by when it was full and kill them for levels/loot. Problem is, after getting a small amount of mobs (30 or so), they all disappear at once. I've seen videos where people have had masses of mobs in this tube, any ideas or suggestions on why mine won't do it?

[progrendon]

de-spawn maybe is the chunk loaded