Data Merge WAS failing to display data field placeholders in either existing documents or new ones. I'm getting a completely Data Merge blank panel when importing a new source file, and when opening an existing document with a linked file, I was only seeing a preview of one of the records.
This persisted through a number of workspace configurations (most including a second monitor) until I broke the Data Merge panel off from the others, and cycled through the panel size presets (the up/down chevrons to the left of the panel title) - only once returned to 'standard' size did the Data Merge panel again display the merge tags, and allow the preview button to be clicked etc