The most common reason that a Reserved Instance discount isn't applying to the instance in the way that you expect is that the terms of the Reserved Instance do not match any instances running on your account. The Reserved Instance that you purchase must match the platform, type, size (or size footprint), region (if applicable), and tenancy of an instance for it to apply. Check the terms of your Reserved Instance and the instance it applies to.

If the Reserved Instance terms do match the instance you intended it to apply to, consider the following additional conditions:

• Reserved Instance discounts are applied differently in an organization's consolidated bill, depending on whether Reserved Instance sharing is turned on or off.
  For more information, see How is the pricing benefit of a Reserved Instance applied across an organization's consolidated bill?
• For size-flexible Reserved Instances, the billing benefit is not necessarily applied to an instance exactly matching the terms of the Reserved Instance before it matches a
  complementary grouping of smaller instances. For example, if you run an m3.large instance and two m3.medium instances, and you purchase a Reserved Instance for an m3.large instance, the Reserved Instance billing benefit might apply to either of these groups of instances. To see which instances are covered by Reserved Instances on your account, use one of the bill reporting options here: How do I tell if my Reserved Instances are being used?