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.