System Administrators (and users with similar custom profiles) in Salesforce may find that when they run an Activity report, they cannot see activities assigned to other users — even though they have the System Administrator profile, which typically grants visibility to all records.
This occurs because Salesforce Activity reports use the Role Hierarchy to control record visibility. In Activity reporting, the Role Hierarchy takes precedence over the System Administrator's "View All Data" permission. This means that even a System Administrator will only see activity records that are accessible based on where their role sits in the org's role hierarchy.
Ensure the System Administrator Role is at the Top of the Role Hierarchy. In Setup, verify that the System Administrator role (or the role assigned to the System Administrator user) is positioned above all other roles in the organization's role hierarchy.
Select the Highest Role When Running the Activity Report: When running an Activity report, make sure to select the highest role available in the report's role hierarchy filter.
Activity Archive Days Limit Activity records created more than 365 days ago are automatically archived in Salesforce and are not included in Activity reports by default. If you need to report on activities older than one year, contact Salesforce Support to request an increase to the Archive Days setting for your organization.
Note: Increasing Archive Days beyond 3 years is generally not recommended. Activity reports that must query very large volumes of archived records may time out before completing.
Please see the below article for more information:
Increasing Archive Days for your Organization
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