Ensure if the timesheet is associated with a Fixed Cost Project. If it is, such timesheets are considered Non-billable and cannot be individually included in an invoice. Instead, a single line item representing the entire project, along with its fixed cost, will be added to the invoice.
As a workaround, you can enter a note in the Notes section of each timesheet indicating that the time log has been invoiced. These details can be exported from the Timesheet Details report.
Books
Online accounting software
for small businesses.