Hi, I got the same kind of problem.
I created a coupon when the customers sign in our newsletters. I excluded the sale products so a customer try to place an order with sale and non-sale products, the coupon didn’t work. But then she try to place another order with just the non sale product she wanted but the coupon still wasn’t working. When I tried to place an order with just 1 non sale product it works properly. Do you have any idea why it’s not working?
