Why doesn't my mob grinder work?

The mob grinder doesn't work, because it is basically what the mobs want, i.e. It is totally up to the mobs whether they will walk ahead and fall into the water stream, which happens, but very slowly.

I want to say that I also encountered this problem, how I fixed it was, I used a soul sand elevator and trapped a villager in a glass chamber just above the hole such that zombies could fall in. This made it so that mobs could fall down. You can use this solution, but it is still not very efficient.(this makes most zombies fall down, but creeper and skeletons don't which ultimately makes the farm ineffective)

I would suggest you to build a spawner mob farm, it is very useful. If you want, you could build this farm, and instead of keeping campfire in the center, you could make a 23 block height pit where you could make a collection chamber to turn it into a xp farm. You could even try this if you are looking for trident drops as well as xp.

I hope this helped. :D

Firstly, make sure that your difficulty isn't peaceful. And then try block out all of the light rays (placing blocks around the edges to not make light come in)

In Minecraft you have a safe zone, that means that mob will not spawn in them (avoid the mob pop in front of you) You need to keep a distance for the mob spawn and wait for spawn

Here's an image from the wiki:

• In your pictures we can see the at the top mob grinder Y : 91 , and inside the structure the platforme for mob spawn shoud be 88 , the spwan area is 24-32

afk platforme Y should be 112- 120 (88 +(24~32)) or 56-64 (88 -(32~24) )

You've built your afk platform too low. You should use this tutorial :

and follow the instructions to build your afk platform at least 100 blocks about the roof of the grinder. It's the height of the afk platform that's the problem, mobs won't spawn if you're too close to the spawning area.