Promotion works fine when a user orders a new subscription. But on subscription upgrade promotion is not applied.
How to apply promotion to an existing subscription?
It is possible to assign promotion to an existing subscription since OBAS 4.5.5. The process is described in the Provider's Guide chapter Applying Promotion to an Existing Subscription.
If your OBAS version is below 4.5.5, please upgrade as described in How to upgrade OBAS system to the latest available version.