The invoices can be generated by:
Click Invoicing
Click Pre Invoicing
Click Pre Invoicing Wizard
Click Yes to the message about Open rounds
Click Next
Select the Period required from the Period dropdown.
Select the oldest date the invoices would be generated on from the date picker.
Click Next
Click Account/location and search for the account you are generating the invoices for.
Click Finish
Check the transactions look correct
If these look to be correct, you can then create the invoice required. Click Create Invoice
This will need to be repeated for next oldest missing invoice date.