MP3 Player won't play all files

My Audiophase MP3 only plays a portion of the stored files.  The weird thing is that the selection of files it plays changes.  For a long time it showed that I had 76 files (there were really about 156).  So I tried manipulating the folders, moving "unknown" songs and albums into other folders.  That helped - next few times I used it it showed I had 147 songs.  I just added ten new songs and now it says I have 55.  When I look at the properties it shows 157 files, and I know for sure that most, if not all, of the now unseen songs have played in the past, and still appear on the directory.  I've only used about half of the 1Gb memory, and nearly all the songs I've loaded I've copied off my own CDs.