MP3 files not showing up


I am total newbie in Linux and Raspberry. I had just recently successfully install Volumio into a 64GB SD card. I also recovered the remaining free space into root to store my music files. I know this is not really recommended but I did successfully and manage to store about 55GB of music files in those space. Most of the files stored are flac or wav. I had copied few Mp3 files into the space and I refresh to library. Funny thing is only 2 mp3 files show up but the rest which I had copied in is not showing. Any suggestion ?