If certain user's timesheets are not appearing in the timesheet export, check the following things:
The user's timesheet(s) for the period are all approved
For most payroll softwares, only approved worked hours will be exported, so if the user's timesheet has not been exported, check all of their hours have been approved. This will be show in the export screen before exporting.
The filters (team / pay period) for the export apply to the user
Check that the user works in the team/locations you are exporting for.
If you are exporting timesheets for a weekly pay period, check that the user's timesheets are also weekly (not monthly).
Check the export screen, and export errors if any appear
Before exporting, all timesheets will appear (including pending) and any potential issues will be highlighted. If the user does not appear in the export screen at all, it is likely one of the two issues above are causing this.