Exporting to PDF/A-1A

When exporting a file to a PDF/A-1A file, the file created is not compliant with the PDFA-1A standard.

When I try to validate the file using Preflight in Adobe Acrobat, or Callas PDFaPilot, the following error shows ups:

“Text cannot be mapped to unicode (1 match on 1 page)”

Anyone knows whats the problem?