One additional item that might help. In the timesheet window, if a task is not showing on the timesheet, the user can click on the Add Row icon and then select “Select from existing assignments”. When they do this a pop-up window appears showing all the projects on which they are assigned. By clicking the triangles on the left of each project and subsequent WBS levels, the resource can drill down into a project, find the tasks they need to have added to their timesheet, and add them.
However, you may not be able to add a task to an earlier timesheet if the task has a later actual start date. I’ve never tried that scenario.