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?
FIX: Resize any textbox containing data merge tags that is overflowing
I encountered the same issue where running a data merge via Data Merge Menu ► Create Merged Document produced the correct merged results, but when exporting via Data Merge Menu ► Export to PDF only a single field was being correctly merged. This is clearly a software bug and I discovered a work around for my case.
Some of my data merge field tags were larger than the text replacing them so the text box was small and the tag actually overflowed the text box. I resized the text box to prevent text overflow and then re-exported with success. The resulting exported pdf was a correctly merged document.
I tested this by resizing select textboxes and only those not overflowing merged correctly.