Minecraft Mob Farm / Mobs Don't Spawn?

I built a parr block under the water layer, a couple of rooms in the underground and a standard MobFarm built with water channels in the underground.
But it spawns no mobs. Does jmd know a solution?

Because normally mobs don't spawn in the water

There must be no light source and you have to stand a little away

and no monsters spawn on water

They're not on water. I just said that the MobFarm is not in the air but in the ground, was stupidly formulated

It was clear that you didn't build it in the air
I mean your driftwood, which brings your monsters to the killing site, if that's permanent there won't spawn on it
otherwise you must be 24 (or 30) to 128 blocks away from your farm to spawn mobs

Do you have mob spawn on?

Mobs don't spawn on water.

Mobs only spawn in the dark.