Some tasks do not appear on the timesheet even though they meet all the timesheet criteria.
In general, timesheets show items that meet the criteria defined in Setup > Timesheets & Hours > Preferences (as described in "Timesheet and Hour Preferences").
In addition to items that meet these criteria, timesheets also show additional items where no hours are logged within 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 only some of them. Items remain on the timesheet based on their last update date, meaning that the ones with the latest update date stay and the others disappear from the timesheet.
To view items on the timesheet that have disappeared: