Minecraft farm not working?

I built a farm monster farm on Skyblock, with trap doors where the monsters run down and are driven down by water, but it doesn't work, it spawns hard on hardly any. Can someone help?

Monsters don't spawn in the water and I don't think on trap doors either, and you don't have to be around otherwise nothing spawns anyway.

You have to be more than 24 blocks away from the fern for something to spawn. (And max 128 blocks away).

How big are the platforms? I guess the water is in such a + shape, and next to it the platforms?