Tasks not showing up in reports

Some completed tasks are not showing up in reports.

I have one report created by a manager to view the completed tasks and activities of her employees during the previous week. It shows that a particular employee had 86 completed tasks last week.

That employee has their own report just for their own completed tasks the previous week. This report shows only 8 completed tasks.

I am an admin and went to recreate the first report, copying the same settings from the Outline and Filters tabs in the report builder. This report shows only the same 8 tasks as the employee's report. The filters are: All Activities, Completed Last Week, Completed Activities, Tasks, Assigned to [selected employees].

The sharing settings for activities are Controlled By Parent and Grant Access Using Hierarchies. The sharing settings for accounts are Public Read/Write and Grant Access Using Hierarchies. We had no hierarchies set up, so I made a simple one that goes Admins > Managers > Employees and nothing has changed.

Looking at the missing tasks, there is no reason for them to be archived or for the employee to be unable to see them. They were created by the employee a little more than a month ago and then completed last week. They are attached to accounts owned by the employee. IsArchived = False.

Another issue that may be related is that the manager's report sometimes does not count completed tasks that are set up as recurring (ex: employee marks task as complete, a new copy of the same tasks is created automatically for the next month, task for this month does not show up as complete on report).