To view a list of missing timesheets for a given period, select the "Timesheet Status" report from the "Timesheet" reports category.

This report allows you to view all timesheets in a given period. To view only draft/unsubmitted timesheets, untick all the fields in the timesheet status filter section except for "Draft". Run the report to see the list of users who have not submitted a timesheet for each week in the date range chosen.