iTunes wont open .mp3 files

I have some .mp3 files on my macbook pro. I played them with quiktime and they work properly, but when i add them to iTunes, either by dragging them or by add files to library, only five of the 17 files are added. Also, when i add the ones that didnt add separately, nothing happens and it is as if i just hadnt done anything. What can i do?