Question about Outlook Express

One of my users has an occasional problem with sending Word documents via Outlook using File -> Share -> E-mail (as Attachment) - it usually works, but often doesn't.  When I looked at the problem this morning, it was also occurring with Excel, but I didn't try PowerPoint.  Outlook is definitely set as the default email application, and I switched it to Apple Mail and back to Outlook with no effect.   Because it's sporadic it doesn't seem to be a preference file problem, but I'll try that next.  Anyone have any other ideas?  Thanks!