the discount doesn't work properly

Good morning to all of you.
I applied a €1 discount to a product.
The discount also works well when I add a second product, but the discount was not applied to the second product at checkout.
The total of the products is not equal to the total of the cart
this is a short video
http://www.cascinaalba.it/varie/discount.mp4
To solve the problem I added a second category (discount) to the product and applied the discount to the category (discount) but in this way no discount is applied
Many thanks to you all

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