Doesn't recognize songs M6SL

Hello.

I've been living with this problem since the very beginning but now I am just fed up.
I have 987 music files on the player, but it recognizes only 770. Which sucks really bad.
As the files are in the player and I have a backup of all the files on my computer, I would like to check which files the player doesn't read and then know why and how to fix it.
But I just don't know how to do that.

Any suggestions?
For the file check.
For what the problem could be [ wrong format, bitrate? ]
What converter to use, to fix this problem.

Cheers, Tom!

[ edit ]

I thought of my own method.
The problem was, there were invisible files [ weren't shown as hidden files ] so the music file count was wrong. I went through all my player music and got rid of those misleading files, so everything is ok now.