Summary:
There could be scenarios where you have multiple permission assigned to a user and on one of those permissions Can View All Report Data is enabled.
 
The user may still be unable to run specific reports for all the users in your company. This is because the report my be disabled in the permission that has Can View All Report Data enabled.  
 
Example: 
– User A has Project Leader and Supervisor permissions assigned. 
– Can View All Report Data has been enabled for the Project Leader permission.
-When the user tries to run the TimeSheet Detail report he can see data of users who are entering time for which he is the project leader.
– If you check the reports tab on the Project Leader permission the TimeSheet detail report is not enabled.
 
Once you enable the TimeSheet Detail report on the Project Leader permission the User A will be able to run the report for everyone.
 
Solution:
 
Make sure you have enabled the required report in the permission which has "Can View All Report Data" enabled.