If you create invoices on the basis of billing periods, that is, if you use a unique date range filter in the Invoice Filters and Options dialog in each billing period, you cannot include unbilled expenses from a previous billing period in the current period unless you change their date to match the current period. As a workaround for this, you can manually adjust the Billable Expenses in the Create Batch Invoice screen for the current period to include the amount from the expenses in the new invoice. Check CORE Help Center for details on this field.
After that, you can mark those expense as Billed in the Expense Entries screen. Alternatively, you can even Batch Update the Invoice Number on the expense to attach it to the new invoice and mark it as Billed without affecting the amount on the invoice (a process known as Progress Billing). CORE only adjusts the markups on the expenses of the invoice in this case and does not change the invoice amount, which you had already adjusted by updating the Billable Expenses.