Users can only see their own time and expense entries in the Time & Expense Reviewer screen and cannot see the entries of other employees.
This can happen if your security permissions for the Time & Expense Reviewer screen restrict you from accessing the time and expenses of projects and employees (or vendors) you do not manage. This restriction is controlled by the security options Allow view time and expense only for the projects I manage and Restrict employee and vendor list to the ones I manage. If you want to access all the time and expenses, ask your Core administrator to un-check these options under the Reviewer module in your Security Profile. Check Core Help for details on customizing security permissions.