My EC2 Reserved Instance isn't applying to my billing—why?

For a Reserved Instance's discount to apply, the following things must be true:

• Your RI must exactly match a running EC2 instance’s characteristics. A running instance must match the type, Availability Zone, platform, and tenancy of your RI exactly.
• The upfront cost for your RI must process successfully. Check the status of your payments on the Payment History page of the Billing and Cost Management console. To retry a failed Reserved Instance payment, contact AWS Support. Failed Reserved Instance purchases from previous billing periods can’t be retried.
• Your Reserved Instance must still be active. When you purchase an RI, you choose a one-year or three-year term. After the term expires, your instance is billed at the On-Demand Instance price. To continue receiving the discount, purchase another Reserved Instance with the same specifications.
  Tip: To avoid gaps in RI discounts, use reservation expiration alerts.
• You must have one Reserved Instance for each instance that you want to receive a discount. Each RI provides the discount to only one running EC2 instance at a time. Any additional running instances are billed at the On-Demand Instance price.