When you go to the RI Optimizer you can only get recommendations for Shared Scope. When you try to filter by Single Scope no recommendations are available.
The recommendations could be missing in the Single scope because that specific subscription’s usage is too inconsistent to guarantee savings. CloudHealth’s optimizer only suggests a Single RI if it’s a "safe buy" (steady 24/7 usage). The Shared scope shows results because it can aggregate that "bursty" usage across your whole enrollment to reach a stable threshold.
You can find more information on how the Optimizer logic differs by scope here: https://techdocs.broadcom.com/us/en/ca-enterprise-software/it-operations-management/cloudhealth/saas/index/working-with-reports-and-recommendation-of-tanzu-cloudhealth-working-with-reports-recommendations/working-with-reports-and-recommendation-of-tanzu-cloudhealth-azure-reservation-management.html