Tax override not working, check charge tax box?

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I have searched some topics but i can't really find an answer. So sorry if I'm asking an obvious question.

I have products with 21% tax an 9% tax. I set up regular tax as 21% and made a product override with a collection filled with 90% of my products (its medical equipment so lower tax)


 First i thought i had to check the box for VAT in the product page as well as having it assigned to the 9% collection, but then i saw on my invoices that the VAT was 30% with some products ( i guess thats 21 + ? ) so i unchecked all the boxes. Now i just got another order and it says 0% tax while it is in the 9% collection and the box is unchecked.. what is the correct way to handle the taxes?

Thanks in advance,