System Administrator is Unable to View All Activities in a Salesforce Report

Within activity reports, the Role Hierarchy trumps the ability of the System Administrator to view all records.

To be able to view all activities within reports, organizations must ensure the Role Hierarchy is set up in a manner where the System Administrator role is above all other roles.  

In addition, when running an activity report, ensure the highest role in the report's role hierarchy is selected. 

Also, keep in mind that activity records created over 365 days ago are archived in the application and are not reportable.  To increase the Archive Days to more than a year past, please contact Salesforce Support to request this feature.  Typically, going beyond 3 years is not recommended as the activity reports might time out if there are too many records to query.

Please see the below article for more information:
Increasing Archive Days for your Organization