This could be a few things...
1 - Verify that the Task in question is set to Reportable. You can view this column in the Work plan by clicking on the Gear Icon, selecting Manage and then searching and selecting the Reportable field.
2 - Another issue could be if the resource was at any time deactivated or replaced at the task or Project. This would set the <Resource>.State (different from <WorkItem>.State) to INACTIVE. To check this, you will need to maximize the Resource Panel and then clicking on the Gear Icon, selecting Manage and then searching and selecting the <Resource>.State field. If the column shows a LOCK, this means the resource is Inactive on the Task and cannot add time (so cannot see task in timesheet). Change in the column to ACTIVE and the Task should appear.