Some tasks don't appear on the timesheet though they meet all the timesheet criteria.
In general timesheets are showing the items meeting the criteria in Setup -> Timesheets & Hours -> Preferences. But there is a concept of additional items which are the items that don't have any hours logged in the range of the timesheet. The number of such items is limited to 45 by default. If there are more than 45 such items the system shows some of them and doesn't show the others. The items stay on the timesheets based on their last update date, i.e. the ones with the latest update date stay and the others drop off.
1. Alter the Timesheet & Hour Preferences for the items that pre-populate on the timesheet so that less items appear on the timesheet.
For more information, see "Timesheet and Hour Preferences."