If you look in the book and press the button: Write down, then a hint will appear on how to brew the potion correctly. After completing all the steps, an effect will occur in the cauldron, and you will need to hover the bottle over the cauldron, hold it without letting go, and it will fill up. You probably just don't read the hints; everything is written there, and they appear at the right moment, as in the screenshots I sent you.
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