I have district-level access, so why I can only see a single school's activity progress?

If you or an administrator has used the group switcher at the top of your dashboard to view a single school group, you will only be able to see information pertaining to that group and any directly related sub-group and will not be able to see the district group’s data. 

Simply use the group switcher once more to navigate back to the district group and you should be all set.