There are two columns that pull in billing details from a ticket. These are ‘invoiced’ and ‘billing status’. ‘Invoiced’ will show the invoice number and link to the invoice that is associated with the ticket and ‘Billing Status’ will show if the ticket is non-billable, needs an invoice, or is invoiced.
Try adding the ‘Billing Status’ column and look for a ticket with the ‘Invoice Required’ status. Open that ticket and select the ‘Make Invoice’ button at the top of the page to send the ticket charges on that ticket to an invoice. After that, the newly created invoice number should appear in that ‘Invoiced’ column.