Sales order won't create sales invoice

HI All,
I have not had the following problem before.
I made a Sales Order.
I saved it and then submitted it.
Instead of then offering me the chance to make a Sales invoice the screen reverted to the list of Sales Orders.
The Sales Order I had just made was listed as “completed”
Now if I go to this Sales Order there is no option to make an invoice.
If I go to Sales Invoices there is no Sales Order available to get info from.
Any idea what has happened?

More Details: https://frappe.github.io/erpnext/user/manual/en/introduction/key-workflows