I use magento 2.2.9 and have huge issues with coupon codes (price rules), it doesn't work properly. I set the pricing rules that are right for me (a certain product category, etc.) and the Code works also propperly.
But when I log in as a user to a shop, dosent catch any more. So i go in as an other user and the code works.
I Go in as the same user on other pc it works.
i delete some products from the basket it works. i add the same prducts back it dosent work.
i go in with other user with the same basket and it works...
Whats happen here 
some wirred conflict in combinatiopn of user,browsercache and products in basket