Why isn't my mob spawner working properly in Minecraft 1.14.3?

This is an awful, ancient design that never worked well and as time progressed, only got worse. It only somewhat works when placed high in the sky and almost doesn't when built on the ground level.

First off, mobs spawn within 128 block radius around the player, so if there are any unlit spaces under ground, mobs will spawn there, and only rarely despawn randomly. That means very few mobs will spawn inside the spawner. Then they must walk at random (rare behavior) into the water channels, and then - unlike in early versions of Minecraft - they'll fight against the current not to fall to their death, and so will only fall when pushed by other mobs in the same stream.

You can get somewhat reasonable (still lousy) drop rates out of this design if you place the kill platform at least 128 blocks above land level (spawn platform accordingly higher; I recommend building over water, reduces chances of death from falling) or at the very least moving the player's afking spot 127 blocks (no higher; otherwise all mobs will despawn!) above the kill platform. Alternatively, with sufficient amount of patience, you can light up ALL the spawning spaces within 128 blocks from the killing floor (...and it's a herculean task) and you'll start seeing reasonable spawn rates. Which will still keep declining as your spawner fills up with spiders congregating under the roof and filling the mob cap.

But what I'd really recommend doing instead is a modern, efficient design like Ilmango's dispenserless.