I am having trouble adding block items such as oak leaves and diamond ore to cauldrons. Same problem occurs with items such as gunpowder and redstone. I am adding items correctly, though, because when I brew something such as normal vodka, everything works fine.

Minecraft version is 1.16.4
Clicking on different surfaces doesn't seem to change anything. I am wondering if the game just thinks that I'm trying to place said blocks, instead of adding them to the cauldron.

Try clicking the Cauldron on top and not on the side. Which Minecraft version are you on?

If this issue still exists, please open it again.

Have you added the recipes correctly that use these ingredients?

I don't see a recipe in your message. You only need to add the ingredients into any recipe and they should then go into the cauldron.

But what are you trying to brew with diamond ore inside it?