u/Deaddemon8201

So ive never built a mob farm before, and i decided to give it a shot. The setup i have right now is a 128 block high tower with a 16x16 room on top, inside the room are four 6x6 platforms in each corner with a two block wide water channel leading to a 2x2 hole. The issue im running into isnt that mobs wont spawn, but mobs not going into the hole. Ive tried putting open trap doors in the hole, but what ends up happening is either the water goes over the holes and mobs just bob up and down, or if I cut the water one block short with a sign the mobs stand on that last block and never go down the hole. Ive tried replacing the trapdoors with signs, and nothing. If I could get any help it would be greatly appreciated. Game version is 1.12 java