I had been using the Data merge function within InDesign CC to create certificates, but recently i found that only one of the 40 place holders were changing in the exported PDF. The preview shows change in all the place holders but the export doesn't show the changing data.
Can you give the steps you took in detail? Were you expecting a document that you had already merged to update after the data source was updated, or were you updating before merging a new copy? Was the placeholder text frame on the document page or the master page?