Hi Brenda,
This issue was reported a few times in previous versions of Acrobat, and the problem was the file path for the problem PDF file that would not combine. If the file path is especially long try renaming/shortening the file path and see if that helps. It could be that you have never seen this before because you've never run into a PDF with an unusually long file path. Another person had some forward slashes in the file path and removing those solved the problem.
Hope this helps,
Dimitri
WindJack Solutions
www.pdfscripting.com
www.windjack.com