What Causes Lack Of Sensation In The Clitoris?

My girlfriend i'm currently with has never been able to orgasm. She has been with two other guys besides myself, and she does not even react when I attempt to stimulate her clit. We talked about this last night, I asked her questions about it because I was concerned, and she said she has no feeling when I try to stimulate her clit. She is also a very small girl like 5'4 and barely over 100 lbs. Is this something we should be concerned about and can it be fixed? Is it a health problem?