User approved some time entries, but they do not show up for billing in the Create Batch Invoice screen.
This can happen if you approved the entries that the employees had submitted to you for payroll only. Employees can do this by selecting Payroll under Type in the Submit Time dialog. Entries submitted in this manner do not show up for billing in the Create Batch Invoice screen when you approve them. To know if this is the case, check the Workflow History of the time entries in the Edit Time Entry > Additional screen. To correct this, you can unsubmit the entries and then approve them. This approves them for both billing and payroll. Check the Core Help for details.