This has one of the following reasons. See which one solves your situation.
1. You may have an outdated login session after setting the discount rule.
As the discount rules are calculated at login, it is possible that the active session on the website was outdated and you did not immediately see the desired result. If you login again from the customer account where this applies after set discount rule, the result will be visible.
2. It is possible that the set discount rule does not match how you logged in.
First check the product in combination with the logged-in user. Go to the 'check product' module and select article, customer and order type. Then click on 'Go'.
Is the 'has discount rules' line set to result 'No'? Then it may mean that the data does not match. Check the set discount rule, with linked target product.
3. The end date of the discount rule has expired, which means the discount rule is no longer active. Go to the discount rule module, click on the name of the discount rule and check the end date: