So, I'm developing a modpack which has custom recipes, and I would like to create a book which shows some of these recipes, the problem is that they won't show up at all

MC version: 1.12.2
Forge Version: 2673
Enchiridion version: 3.3.3
Crafttweaker version: 4.1.8

I'm getting similar... except even some recipes that I haven't changed with Crafttweaker will not show up. For example, the vanilla crafting table, which only costs 4 planks, will not show up.

Furnace recipes that I've added show up fine, though.

I don't know if that's the same issue
however the dev seems really inactive about this mod...

the item isn't showing up?
Enchiridion has built-in support only for vanilla crafting bench recipes.
"Add recipe" gives you an item selection field.
You point it to the item, and it'll show that item's crafting recipe.
I don't know whether it's been fixed or not,
but there (at least was) an issue with the aditional settings of elements being outside the window.
So turn Minecraft's gui scaling down to see if the setting you're trying to do was off the screen.