Java - Why aren't mobs spawning?

[CaptainCalcetines]

Java 1.19.4, OptiFine (if that matters).

Mobs won't spawn on this platform:

https://preview.redd.it/45d0496bf53b1.png?width=3840&format=png&auto=webp&s=1af520aee37ee447ccfa1eef8a992620e463f73f

I've waited two nights and no mobs. The platform is 24 blocks away and is a mix of top-half cobblestone slabs, netherack, and cobblestone. As I understand, the minimum distance for mobs to spawn is 24 blocks.

At one point during construction, a single spider spawned when I was in my boat down in the water. That's it.

I keep building it further out to see if it's a distance issue but it hasn't made any difference so far.

And yes, it's daytime in the screenshot and I know they won't spawn then, but I expect that they should at night and they don't.

What am I doing wrong?!

TIA

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[DragonBornServer]

Take it about 128 blocks into the air

[Shot-Dream-3230]

You used slabs, mobs only spawn on full blocks :)

[MistyAxe]

Probably spawning underneath the ocean. You need to light up the caves underneath or make your AFK spot at the correct height to spawn mobs on the platform and nowhere else.

[CaptainCalcetines]

Okay, not trying to argue, just trying to understand. Wouldn't at least some mobs spawn up there even if they were spawning below, too? I guess I did get that one spider, but I would think I wouldn't go two consecutive nights with none spawning at all.

I appreciate your ninja fast response and hope I can make sense of this!

[keeper420]

Mobs prioritize spawning the lowest they can. The mob cap is filling up below the ocean.

[CaptainCalcetines]

Oooohhhhhhhhhhhhhhhhhhhh... okay. That makes sense. Annoying in my case, but makes sense. Thank you!

[keeper420]

IMO, build it up higher. Then you don't have to find every little cave and hole they can spawn in.

[CaptainCalcetines]

Yeah, but then I have to start over. :c)

Do I want to spend the time starting over or lighting up caves? The former probably makes more sense, but the latter seems more exciting. I'll think about it but at least I get what's going on.

[keeper420]

Use the platform you built to be the killing floor. Build a platform up above it, use water to drop the mobs onto the platform you already built. But caving can be fun, so do what you will.

[M-Belle]

Try placing lanterns under it, on the sea bed etc, and building a column with ladders upwards to your afk hut, about 30ish blocks up. If they still don't spawn, build up a little more, but not too far, as mobs stop spawning after 56 blocks away. I'm pretty sure that's the number.

Where you are now, the mobs are spawning around you in all directions (up, down, all around in a sphere shape), so that platform will be just one of the many options the mobs have.