Sort order in library window suddenly not working

I have been using EndNote 5 for years now.  Just recently I noticed that the references in my library window are not sorting correctly.  They are sorted properly by the last name of the first author, but then they are not sorted by year.  So if I have a bunch of references where the last name of the first author is the same, they are no longer ordered by year.  If I try to sort them manually using the “References - Sort” command, they still do not sort correctly.  Any suggestions?  I have around 7000 references in my library.