Sounds like a spooler memory problem to me.  The article at http://ddisoftware.com/tech/articles/april-2010-understanding-your-photo-printer/ should help because it has a section related to prints not printing.  Follow the links there and you'll find advice on how to deal with memory issues, particularly if you are printing large prints or a large number of prints.  You weren't very specific about what happened but it sounds like QU sent the prints (it showed them processing) but they never appeared.  That's a classic Windows print spooler memory (RAM or hard drive space) failure.

Mike