It's a bit outdated but for the life of me I can't figure out why it won't work. I'm pretty sure I'm standing far enough away (about 22-40 units over and about 20 units below). The water dispensers are working fine---moving the mobs into the channels which carry them to the drop zone. The problem is that just not enough mobs spawn. I've also tried all sorts of different render distances and that didn't help. Farm is built high up on an island (lowest spawn area is at about 190y and the island floor is around 60y). Images are attached. Please help!
View of spawner from island ground level
Approximate waiting position (can walk backward even further)
One cut-away of a quadrant of the spawner
Water operating correctly
Quadrant of spawner fully enclosed (looks pretty dark)