First let me clarify that I am not trying to make a blanket statement for all women. Everything I write is my opinion.
It's not a problem with your girlfriend or your penis. It's perfectly normal for a woman to only reach orgasm from clitoral stimulation. I know it's possible to get there other ways, but most of what I read seems to say the same thing. You can almost think of her clitoris like the head of your penis. You can rub the shaft all day and it feels good, but not OMG wow,great!
Eventually some of us also get sore and uncomfortable from penetration and kind of give up on the orgasm. If your girlfriend is always pointing you to her clitoris then that is probably the type of stimulation she requires to orgasm and I would advise listening to her. Each of us are different.
You might have read this article from wiki, but remember you can't always count on everything on that site to be the facts.
Female ejaculation - Wikipedia, the free encyclopedia