The historical Time off Bookings my not show all the timeoff bookings of all the users. This usually happens when the historical Timeoff bookings page is not being utilitised properlly.
Click on Administration tab in the top menu
Click on Historical Time offs under Shortcuts in the left side menu.
In the Historical Time offs, click on the root department under the Departments section in the right panel and check the box beside Include sub-departments which will reveal all the historical Time off bookings.
On the other hand, clicking on any one of the random departments will show only the Time off booking of that particular department which will have only limited number of historical bookings.