itunes wont add certain mp3 files

itunes wont add certain mp3 files, I tried to drag an album into itunes and only half of the songs imported, the others wont. when i *right click* get info the only difference between the files that copied and the files that didnt is the ones that copied have the album cover and under more info have the album, title,musical genre ect. The files that wont copy has a itunes symbol MP3 picture and under more info only says the last time opened. What do i do to import the files to itunes, please help!

Thankyou.