You're facing an issue with taxes not showing up automatically on your invoices even though you've set default taxes. This can be pretty tricky sometimes, so let's dive in. In Odoo, even if you've set default taxes, they might not auto-populate if some configurations are off. The problem might be with the fiscal position linked to the customer or the product configuration itself.

First, check if a fiscal position is the problem. Fiscal positions in Odoo map default taxes to specific customer situations where different taxes should apply, like based on region. If there's a fiscal position on your order that doesn't map teh taxes right, your default tax won't apply. Head to the sales menu, under the customers menu, and check the fiscal position for this customer. Make sure there's no mismatch stopping taxes from popping up.

Also, don't forget to look at the product setup. Navigate to the product form, go to the sales tab, and make sure the taxes are correctly defined there. Sometimes users forget to set default sales taxes on the product template, leading to the issue you're seeing.

These settings need to be in sync for taxes to apply automatically. Btw, if these checks don't solve it, you might have a deeper configuration problem or a customization affecting default taxes. Odoo's logic here can be a bit odd, so it might take some time to perfect it.