Problem with Mail Merge to PDF

Hi there

I'm having problems with the mail merge to PDF feature of Acrobat XI, in that the process fails part way through and displays the error message 'Acrobat PDFMaker was not able to mail merge the document':

This process used to work, and still works for one of my colleagues. However I have tried using different PCs (32 bit and 64 bit Windows 7), different versions of Word (2010 and 2013) and different types of data source (Excel and Access) without success.

I would really appreciate any further suggestions.

Thanks

Rianne