On December 26, 2015 I imported 25 CD (stored in a NAS) in a library and made a back up. In that library I had 3550 Albums.
Today, January 25 2016, I accessed for the first time since then to the Library and it shows olnly 3459 Albums: it is not the first time it happens so I learned that every time I add Albums I must also make a library back up. I think that something does not work well.