Writer: generate file name from Database - no options

I have a form letter in Writer that I have been using for years to create individual merged documents. There is an option to Print > (Output) file > Save as individual documents with a tick (check) box to select Generate file name from Database (in this case, an Excel spreadsheet). However, now the Field box is blank and when I click on the down arrow there are no options to select any fields from the spreadsheet. How can I get it to show the fields so I can select one? It was fine until the last time I ran a mail merge (using data from the same spreadsheet) about two weeks ago.

10 Sep: I really need to sort this out as I’ve been asked to send an adapted version of this document to someone else to use in a similar way. Anyone?