Some promotions are only applicable to certain products. Therefore a particular coupon code or promotion might not be applicable to items in your cart.
For example, promotions do not apply for some skus that are heavily discounted already, some of them being build your own products or overage sale products (varies from time to time). Drop Ship products are also excluded from promotions.
In such cases, the promotion discount or offer will be applied only to the eligible skus in your cart.