A project is not showing up in the Create Batch Invoice screen and so the user cannot bill it.
This happens when there is nothing to bill for the project in the specified date range because there might not be any billable, approved time and expenses for it. If you turn on the project rule 'Always show in Create Batch Invoice screen', you will always see this project for billing. This rule is usually set for project phases so that they appear on the invoice even if there is no charge for it.
If you don't have time and expenses associated with a project, you can also use the Manual Invoice option to bill that project.