How was the original data source created?

As an immediate work around, use the existing data source with a Directory
type mail merge main document in which you insert a one row table into the
cells of which you insert the mergefields.  Then when you execute that merge
to a new document, that document will contain a table with a row of data for
each record in the date source.  Insert a row at the top of the table into
which you type the names of the fields and then add any records or edit the
data in that table and save it and then use it as a data source for the
merge that you originally wanted to create.