AWS backups conformance reports are empty

I'm trying to generate conformance reports for our backup plans but the reports generated appear to be empty.

I have a backup plan for RDS to make snapshots, I have a framework deployed, this framework appears in the state "Partially active" If I check the documentation says that I have to activate the tracking of all backup resources and the tracking of AWS Config resources in AWS config and these are active, in fact, the "Resource Tracking status" shows every item with green tick and "OK".

I have Control compliance and Resource compliance reports. According to the framework there are four compliant controls. According to AWS Cnfig the rules associated to these controls have several resources. The report finishes as "Completed" But the report stored in S3 appears empty. There's a Pop-up warning about the "Partially Active" state of the framework.

The reports are empty only with this content:

{
  "reportItems": []
}

Why this can be happening? I do not see any error or possible cause.

How I can know what resources should I track in AWS Config for the framework in AWS Backups to become "Active"? (In case this is the problem)