If Product Discount has already applied a discount to a product, it will skip any additional discounts.
If a price being discounted results in the price being calculated as less than zero, the discount will be ignored. For example, a $10 item is included in a sale that discounts the price by $12. If the discount were applied, the price of the $10 item would become -$2. Shopify does not allow a product to be sold at a negative price. In other words, a negative price would mean you are paying the customer to buy the item.
On some discounts, especially low priced items with a low discount value, it may appear that a discount is not being applied. For example, a product priced at $1.50, a 10% discount applied, and override the cents to .79 will not appear to be discounted.
Here's why! The discount on the $1.50 product will be calculated, reducing the price by $0.15 to $1.35. When the cents override is applied, it will make the price become $1.79. The price would become higher than the original price. Therefore, the discount is ignored.
Another issue could be If you are using apps that sync information or modify products, like Oberlo, you may experience odd discount amounts being applied or products not being discounted at all. Unfortunately, there is no solution for this and you will have to choose to use one app or the other.