A coupon will always apply the correct discount amount.
The only reason a discount may not be fully applied is if an item/product in your cart isn't applicable.
For example, if the coupon is entered and there are products in your cart that are already on sale, these particular products will not be further discounted.
Another example, If a furniture item is in your cart and that particular coupon excludes furniture.
Only the non-furniture items will be discounted.