Very slow mob spawns in SMP mob farm on modded server

I run a very small SMP server running Forge 1.12 hosted through MCProHosting. I have a very few mods installed: StorageDrawers, WAILA and JEI.

I built the mob farm in this video.

The spawn rates are REALLY low, only a couple mobs in over 10 minutes. I've done a lot of research but can't find any useful information. I'm looking for advice on how to make this thing work. I've read that for various reasons, mob farms are just much worse on SMP. Here's some useful information:

• I wait for spawns between 24 and 128 blocks from the spawning areas.
• I turned the server's render distance to 10. Helped a LITTLE bit.
• I used the fill command to ensure NO unlit caves are within a 128 block radius
• The server is on the Hard difficulty
• The spawn rate is very low even when I'm alone on the server.

Any help is appreciated.

Thanks!

Edit

I downloaded the world and tested the trap in single player. To my surprise, it didn't work!! That's kind of good news. That means maybe it's possible to have this work on the server once I figure out what's wrong with it.

I have recreated this spawner on a test world. I initially built it on a spigot server. I played it as single player and also loaded it in a vanilla server. The spawn rates were substantial all 3 ways.

I can't speak to the possibility that your mods could be causing the issue you are facing though.

Note: A few times, the redstone circuits glitched. I had the clock at the top freeze which caused the water to just stay out which killed spawning. I also had the elevator circuit freeze which caused the items to back up into the hoppers and never come up to the surface. In these cases, the circuits had to be manually reset.

To force the mobs to spawn only on the intended platforms, I filled in most caves and lit the surface for 128 blocks in all directions. You said you filled the caves, if you have not lit the surface, I recommend it. Before I lit the surface, the spawn rate was a bit lower.

I took this screenshot when it was running on the vanilla server. As you can see, every time the water turns off, several mobs spawn. To get this picture, I used a hacked client with mobESP and freecam. MobESP highlights the mobs and freecam allowed the screenshot to be taken from underground like this.

View edit below before downloading this version
I uploaded this world to mediafire. It is named Spawner.zip and it is 5.52MB. It can be downloaded here. I recommend you download it and run it temporarily on your server to see if your mods are causing the problem or if it is something else.

Edit:

I recreated this in an empty world out of glass for a smaller world size and a way to view it. You can see the item stream headed up on the left side of the image.

Download this world here. It is named Spawner2.zip and is 330.13KB. Use this temporarily, just to test if the spawner design will work on your server. To note, the spawner works best at night. Use /time set night when testing as I left the day cycle functioning.